Monday, December 23, 2013
Prospecting for Gemstones
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| Searching for & finding gold in the Silver Crown district in Wyoming in 2012. |
Years ago, I worked for the Wyoming Geological Survey at the University of Wyoming in Laramie. While working on a research project with the Department of Engineering in 1980, I began to search for disseminated gold and base metal deposits for the department. The UW engineers were interested in conducting metallurgical and chemical studies for metal extraction in disseminated deposits.
Two places I thought had very good potential for this project included the Rattlesnake Hills in central Wyoming, where I discovered an previously unknown gold district and investigated a large jasper deposit, and the other was the Copper King mine in the Silver Crown district where a sizable gold-copper resource occurred as a deeply eroded porphyry copper deposit of Proterozoic age (about 1.4 billion years old).
I left the Wyoming Geological Survey in 2007 because politics in the governor's office stunk and the smell spread to the Wyoming Geological Survey's director. I continued conducting my own research and consulting for various mining companies. Here I am (see above), working for Saratoga Gold searching for additional gold resources at their 2 million ounce gold deposit in the Silver Crown district. And I believe I found a continuation of their ore deposit that had been down-dropped along a fault and found another potential target for the company. Not far from here, I identified 50 cryptovolcanic structures that are likely kimberlite pipes for an Australian diamond company (DiamonEx). Just west of those, I found a few diamond deposits from 1977 through 1980, in the State Line district where several other diamond deposits had already been found.
It is interesting that the deposit I standing on in the above photo is deeply eroded based on geology - and so are the kimberlites to the west and the cryptovolcanic structures in between. What this means to you as a prospector, you might take a gold pan, go out on edge of Cheyenne, and start panning for gold and diamonds. There are likely both in streams draining from the Laramie Range to the west of Cheyenne. So why hasn't anyone done this before? Good Question.
Sunday, December 16, 2012
ROCK HOUNDING WYOMING
Agates, Jaspers, Jasperoids, Chalcedony, Quartz, Quartz Pseudomorphs after Jade, Fossil Wood, Petrified Wood and much more were identified in Wyoming and summarized in a book on World Gemstone Deposits
by W. Dan Hausel and Wayne Sutherland. This blog provides you with a
brief discussion about all of these great lapidary and rock hound
minerals and their locations. Over 30 years, the Gemhunter found many
interesting locations for gold, diamonds, and colored gemstones - find
out more by visiting my facebook page and my website at the GemHunter on Facebook
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| A few of the minerals collected from Wyoming and cabbed. |
Tuesday, April 19, 2011
Crystalline & Cryptocrystalline Quartz
Quartz (SiO2) [Agate, jasper, chalcedony]
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| Quartz crystals (clockwise from bottom left). Transparent hexagonal quartz crystals capped by a hexagonal pyramid from Hot Springs, Arkansas, smoky quartz from the State Line, Wyoming, amethyst, translucent quartz, and bottom right, smoky quartz from Big Creek, WY. |
When we discuss quartz, chalcedony, agate and jasper, we are talking about the same material. All are formed of silica (SiO2) and each is different from the other only because of crystallinity or color. For example, quartz
is the most common constituent of the earth’s crust and when found in
veins, it can be coarse crystalline with distinct crystals or massive
with no distinct crystals - most often it is massive.
When
we speak of massive we mean that we cannot distinguish any distinct
crystal form with the naked eye - and this is typical of rock quartz,
jasper and agate. When found in hand specimen, this could be termed
simply massive quartz, quartz, rock crystal or chalcedony.
If the quartz
is found in crystalline form, something that is attractive to mineral
collectors, rock hounds, and new-age crystal healers, the quartz often
occurs as hexagonal (6-sided) prisms that are almost always capped by a
hexagonal pyramid.
Less
commonly, quartz is found as doubly terminated crystals. This means the
quartz crystal has distinct hexagonal prisms at both ends of the
crystal. By the way, I met a person years ago who had a round tower in
their house filled with quartz crystals for healing. Scientifically,
there is no way that quartz can heal a person unless it is psychosomatic
– it is like putting your trust in a politician not to spend your
money. Quartz is typically found as (1) rock crystal, (2) amethyst, (3) citrine, (4) smoky, (5) rose, (6) chatoyant quartz, or (7) crystalline quartz.
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| Milky quartz vein (massive quartz) in Mary Ellen gold mine, WY. When this rock weathers out, it forms massive pieces of quartz referred to as rock crystal. |
Hardness
7.0
(Mohs Scale). This is just a little harder than your car’s windshield –
this is why it gets so pitted if you live in areas with blowing sand
like Arizona.
Color
Quartz
is found in a multitude of colors that include colorless, white, red,
orange, yellow, gray, brown, black, lavender, violet, purple, pink, blue
or green. Essentially every color is seen in quartz.
In
its chemically pure form, quartz is usually colorless or white. As with
other gemstones, small amounts of chemical impurities produce a variety
of colors. Quartz with impurities is quite common, and the range of possible colors spans the spectrum.
In
its various valence states, iron produces many colors in quartz, the
most important being the purple to red-violet of amethyst and the
lemon-yellow of citrine. Iron is also responsible for reds, yellows and
browns typical of many agates. Iron gives carnelian and jasper their
brownish-reds and oranges. Iron or manganese produces browns and blacks
in dendritic agates. Traces of titanium cause the characteristic pink of
rose quartz and nickel gives chrysoprase its
apple-green color (Schumann 1997). Other chemical colorants are
possible. Mineral inclusions in coarsely crystalline quartz and in cryptocrystalline varieties may also impart colors as well as optical effects such as chatoyancy.
Specific Gravity
2.65 (more than twice as heavy as water).
Cleavage
Generally
none. In other words, you are not going to be able to break quartz
along distinct planes like you would diamond. If you do strike quartz
with a chisel and hammer to try to cleave, it will break forming
conchoidal fracture (just like broken glass).
Cleavage
reminds me of attractive women, but it also brings back memories of a
family from Cheyenne who visited my office at the University of Wyoming
thinking they had found the largest diamond on earth: the size of a football. They had been told by a gemologist in Cheyenne that it was a diamond, but needed to have it verified by someone else (now that was a very good suggestion). When they brought it into my office, I knew immediately it was rock crystal, but wanted to let them down slowly. So I toyed with them a little and then got out a diamond chip and cut the quartz with little effort. They were visibly disturbed that I would dare scratch the diamond that would pay for their mansions and allow their off-spring to live a life of luxury. After I pointed out that if it was diamond, I would have had a very difficult time cutting the stone, they relaxed and the sweat stopper pouring from their brows.
BUT
– they were not done. On our staff, we had a super person and
geologist, but was somewhat of a klutz. Ray unfortunately ran into one
of the most disgusting politicians in Wyoming who drove him to an early
grave a few years later. Anyway, Ray was a very good field geologist,
but he was not so good at mineralogy.
A
few weeks later, this family came back to the university and asked to
see Ray and avoided me. Ray told me what was going on and I said he
should go ahead and give these people a third opinion. But what these
people didn’t know was that Ray had no experience with diamonds.
During this meeting, Ray told the family he wanted to look at the
Crystal a little closer. As he was carrying the stone to this lab, he
dropped it. I wish I could have seen everyone’s expressions – it would
have been priceless. Here was a 'diamond' much larger than the 3,106 carat Cullinun (the largest ever found), and it was dropped on the floor shattering into dozens of pieces. Ray’s response simply was, “well, it has conchoidal fracture, so it’s not a diamond”. Ray was partially right.
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| One day, while my field partner Wayne Sutherland and I were driving in the middle of no-where in the Granite Mountains, we came across the remains of a two-track road, likely last used by a 19th century wagon train. We had been drinking too much coffee and stopped in a low spot to relieve our selves. As I looked down, I was surprised to see a tumbled and polished Sweetwater agate. Next I found a couple of more, and then Wayne started finding some. They were all within throwing distance of the 2-track road. Who threw them there and why is a mystery, but it goes to show you, when nature calls, it is for a reason. |
Crystal System & Habit
Trigonal,
hexagonal-rhombohedral. The common habit of quartz is anhedral
(formless) masses and well-formed hexagonal prisms capped by one or two
hexagonal pyramids or terminated by rhombohedra.
Luster
vitreous
Electrical
Strongly piezoelectric and pyroelectric.
Mineralogy
Crystal Habit.
Quartz
crystallizes in the trigonal system (Berry and Mason, 1968; Hausel
1986, 2005, 2009; Kievlenko, 2003;). Coarsely crystalline quartz most
often is found as anhedral grains and masses in various rock types.
Large crystals usually form singly terminated hexagonal prisms that are
often striated perpendicular to the c-axis (the c-axis is an optic axis that is parallel to the hexagonal prism)
and are capped by a pyramid or rhombohedra. Such prisms typically range
from microscopic to crystals weighing several hundred pounds.
Periodically, doubly terminated prisms are found that are bipyramidal,
but these are less common (Mason and Berry, 1968).
Amethyst
typically is dominated by forms with rhombohedral faces, whereas prism
faces are absent, or at best, poorly developed. This habit is typical of
crystals that have grown simultaneously on the walls of open cavities
(Lowell and Koivula, 2004).
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| Twinned quartz - note smaller crystal growing from larger. |
Twinning.
Quartz crystals are sometimes twinned with two different crystals growing together.
Size.
Crystals
usually vary from microscopic to specimens weighing several hundred
pounds. On the extreme end, rock crystals have been found that weigh
many tonnes (Schuman 1997). Pyramidal amethyst crystals up to 10 inches
in diameter are reported from the vicinity of Thunder Bay, Ontario,
although facetable gem material is usually only found within a small
portion of such large crystals (Kievlenko 2003).
Fluorescence
Quartz is not fluorescent or luminescent (Hurlbut and Switzer 1979).
Geochemistry
Quartz (SiO2)
is the most common rock-forming mineral in the earth’s crust and occurs
in both coarsely-crystalline and microcrystalline forms. In pure form,
it is composed of 46.7% silicon and 53.3% oxygen (Bauer 1968; Hurlbut
and Klein 1977). Quartz is usually very pure SiO2 (Mason and
Berry 1968; Hurlbut and Klein 1977). Quartz is infusible, and generally
considered insoluble except in hydrofluoric acid (Bauer 1968; Hurlbut
and Klein 1977). However, Deer, Howie, and Zussman (1971) note that the
presence of either NaOH or NaCl in water accelerates both solution and
crystallization of quartz by several orders of magnitude over that of
pure water, which has ramifications relating to its geological
occurrence.
In
its chemically pure form, quartz is usually colorless or white. As with
other gemstones, small amounts of chemical impurities produce a variety
of colors. Quartz with impurities is quite common, and the range of
possible colors spans the spectrum.
Mineral Associations & Inclusions
Quartz,
being a common constituent of many rock types, is found in association
with a wide variety of minerals. These include feldspar, muscovite,
biotite, chlorite, tourmaline, beryl, hematite, iolite, barite,
fluorite, nephrite jade, gold, sulfides and others too numerous to
mention (Ford 1949; Bauer 1968; Keller 1990; Hausel and Sutherland 2000;
Garland 2004). Many of these associated minerals often occur as
inclusions within quartz. The diversity of the inclusions is a striking
feature of quartz (Gübelin and Erni 2000). The introduction of
silica-rich fluids into the presence of other minerals may be
responsible for some inclusions, or the foreign inclusions may
crystallize somewhat simultaneously with the quartz. Many of the
better-known inclusions give rise to type names that are often listed as
subspecies of gem quartz.
Reddish-brown
hematite flakes, or more commonly flakes of green fuchsite mica produce
the glittery metallic appearance (aventurescence) and colors
characteristic of the coarsely crystalline quartz variety known as
aventurine (Schumann 1997). Rutilated quartz (quartz containing
elongated mineral inclusions of rutile needles) most commonly exhibits
yellowish-brown to golden-yellow colors, but also has produced
coppery-red and silver-gray colors. Although rutile in quartz is
relatively common, rutile in highly transparent quartz is rare. The
abundance and parallel alignment of fine acicular rutile inclusions
sufficient to produce chatoyancy in transparent quartz is rare (Koivula
and Tannous 2004a).
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| Beautiful slab of banded agate from the Jay Sundberg collection, Rawlins, Wyoming. |
Gold has always been associated with quartz to the degree that early prospectors almost always looked for quartz
in their search for the precious metal. Gold in quartz, most commonly
in milky quartz, is cut and polished in a variety of shapes and sizes,
primarily for use as jewelry. Striking specimens of gold-in-quartz have
been produced from the Badger mine, Mariposa County, California, from
the Sixteen-to-One mine in Sierra County, California, and from Australia
(Laurs 2005c).
Less
commonly found within quartz include covellite, which produces an
electric pink schiller in both smoky and colorless quartz (Quinn and
McClure 2005), and pezzottaite (a Cs-, Li-rich member of the beryl
group) that shows as a pink-zoned inclusion in colorless quartz (Koivula
and Tannous 2005). Koivula, Tannous, and DeGhionno (2004) reported
colorless quartz appears blue due to the copper content of abundant
ajoite fibers, strawberry-red due to numerous transparent to translucent
hematite inclusions, and green due to nickel content of included
népuoite. Lead-gray metallic molybdenite inclusions have been found
within large rock crystals recovered from the Confianza mine, Chile
(Koivula and Tannous 2004d). Well-formed graphite cylinders and
botryoidal masses have been found in rock crystal in Brazil (Hyrsl
2004b).
Gas-fluid
inclusions are common in quartz (Bauer 1968). Most are microscopic, but
large visible ones also occur. Analysis of gas-fluid inclusions
provides a wealth of information about the pressure, temperature, and
chemical environment under which quartz crystallized. Such inclusions
are always present in amethyst with the greatest abundance near the base
of the crystal. These are primarily formed of gaseous and liquid
carbonic acids (Kievlenko 2003). Carbon dioxide, water, and common salt
are also found as inclusions (Bauer 1968), as are natural petroleum
fluids, gases, and solids (Koivula and Tannous 2004b).
Quartz crystals hosting both fluid and solid natural petroleum inclusions were reported by Koivula and Tannous (2004b) in
Baluchistan, Pakistan. Yellow to brownish-yellow and clear fluid
inclusions generally consist of natural petroleum, methane gas, and
water. A yellow to orange or blue luminescence under long-wave
ultra-violet light is characteristic of fluid petroleum inclusions. The
darker bituminous, brown to black amorphous solid petroleum-derived
materials are referred to as asphaltite. Doubly-terminate quartz
crystals from Herkimer County, New York have been known to host
amorphous asphaltite hydrocarbons as inclusions. These originate within
silicified dolomites, and appear to have developed during digenesis of
the host rock. Hydrocarbon inclusions are also abundant in Sichuan
Province, China, but the Herkimer rock crystals are the best known
(Hyrsl 2004).
Varieties
Rock Crystal.
Rock
crystal is essentially colorless quartz. Because of its low refractive
index (the amount of light that is broken up to produce fire or a
rainbow of colors) and glassy appearance it is seldom used as a
gemstone. It exhibits characteristics similar to those of synthetic
glass – thus it has few properties that would make it attractive for
adornment. Even so, colorless raw quartz crystals are periodically used
in earrings and necklaces due to the characteristic hexagonal prisms
that are visually interesting. Beautifully crystallized quartz crystal
prisms are attractive for museums and collectors. The recent
nonscientific practice of crystal therapy has resulted in increased
interests in quartz crystals along with increased prices.
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| Amethyst in ring |
Amethyst. Amethyst is colored quartz that ranges from mauve to deep violet. The coloring agent for amethyst is small amounts of Fe3+
that is distributed in the crystal structure in layers typically
parallel to the rhombohedral faces. Thus in many cut stones clear bands
of quartz may be seen alternating with colored bands (Hurlbut and
Switzer 1979). Amethyst has been one of the more popular gemstones
throughout history and is even mentioned in the Bible as one of 12
sacred stones. It is generally considered the most valuable of all the
quartz gemstones.
Gems
of similar color were once included together in a single group
centuries ago, such as amethyst and sapphire. This is the origin of the
term oriental amethyst that
has been applied to similar colored sapphire. The color in amethyst can
be modified through heat treatment. At temperatures of 250 to 350oC,
the stone will become colorless; however, the color is restored by
radioactive irradiation. At higher temperatures (450 to 500oC)
the color is irreversibly damaged and the mineral may take on a
reddish-brown to yellow-citrine color. The violet color seen in amethyst
will form following radioactive bombardment (ionization) causing trace
iron in the crystal to lose an electron and change from trivalent to the
tetravalent state. Higher-temperature thermal heating will destroy both
Fe3+ and Fe2+ centers in the quartz yielding a
greenish to yellowish color in the crystal (Kievlenko 2003). In nature,
amethyst is thought to have a hydrothermal genesis.
Citrine
Citrine
is a pale- to dark-yellow, brownish-yellow, or honey-yellow quartz with
a russet tint named after its resemblance to citrus lemons. It has been
mistaken for topaz particularly in the ancient past and has erroneously
been given additional names such as ‘topaz quartz’. Even so,
citrine can be distinguished in both the faceted and natural form from
topaz by index of refraction and specific gravity. In hand specimen,
the crystal habit and cleavage is used to distinguish one from the
other. Cipriani and Borelli (1986) report that the specific gravity for
citrine (and feldspar gems) are the lowest for yellow, transparent
gemstones, whereas topaz is noticeably higher. The luster is slightly
inferior to topaz and topaz may show signs of incipient cleavage that
would be non-existent in the citrine.
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| Wiggins Fork citrine, Wyoming |
The
coloring agent for citrine is iron. Citrine can be produced by heat
treatment of amethyst at high temperatures. Almost all citrines derived
in this manner will have reddish tint that tends to contrast with the
predominant pale-yellow in most natural citrines (Schumann 1997).
According to Hurlbut and Switzer (1979), heat treated citrine will
exhibit color banding typical in amethyst and lack slight pleochroism
seen in natural citrine. In contrast, Cipriani and Borelli (1986) report
that natural citrine exhibits patches and color bands similar to
amethyst, although the zoning in citrine is less obvious. Citrine
exhibits a nice vitreous luster and is generally free of mineral
inclusions.
Smoky Quartz.
Smoky
quartz varies from black to brown to smoky yellow and grades into
citrine. The dark color of smoky quartz is thought to be the result of
radioactive damage during exposure to radiation. Upon heating, smoky
quartz will turn colorless, and will return smoky upon exposure to
radiation. Rutile needles are common inclusions in smoky quartz
(Schumann 1997). The best-known locality is that of the Swiss Alps,
where veins have yielded many tons of beautiful crystals. Other notable
localities are Russia, Brazil, Madagascar, and Scotland. In the US,
smoky quartz has been reported in the Pikes Peak region of Colorado, and
at various localities in Maine and New Hampshire.
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| Rose quartz |
Rose Quartz.
Rose
quartz often occurs as coarse-crystalline anhedral quartz that varies
from pale pink to deep rose-red, which often fadesupon exposure to
sunlight. Rose quartz is seldom transparent and instead is turbid. Its
color is thought to be due to the trace titanium. In some rose quartz,
microscopic needles of rutile are found that are oriented in three
directions at 120o from one another and at right angles to
the c-axis. When manufactured into a cabochon with proper orientation,
the mineral will produce a distinct 6-rayed star due to light being
reflected from the rutile needles.
Chatoyant Quartz
There are several minor ornamental stones of chatoyant quartz. Such quartz contains parallel fibrous mineral inclusions that exhibit wavy reflections as they are rotated in light (Hurlbut and Klein 1977). Rather than fibrous mineral inclusions, Koivula and Tannous (2004c) associate chatoyancy with “thin hollow tubes resulting from growth blockage (growth tubes) and/or post-growth dissolution features (etch tubes)”. Chatoyancy is also seen in Tiger’s Eye, a variety of cryptocrystalline quartz.
There are several minor ornamental stones of chatoyant quartz. Such quartz contains parallel fibrous mineral inclusions that exhibit wavy reflections as they are rotated in light (Hurlbut and Klein 1977). Rather than fibrous mineral inclusions, Koivula and Tannous (2004c) associate chatoyancy with “thin hollow tubes resulting from growth blockage (growth tubes) and/or post-growth dissolution features (etch tubes)”. Chatoyancy is also seen in Tiger’s Eye, a variety of cryptocrystalline quartz.
Cryptocrystalline Quartz (Chalcedony)
Cryptocrystalline quartz includes many varieties of ordinary chalcedony
as well as the extraordinary agate, carnelias, chrysoprase, onyx, sard,
jasper, chert and flint that are so loved by lapidaries and rock
hounds.
Mineralogy
Crystal Habit. Cryptocrystalline
quartz crystallizes in the trigonal system and is separated into two
types: (1) fibrous and (2) granular. These two may occur in the same
deposit and grade into each other. Although the characteristics of each
are often quite distinct, in many stones the differences are
indistinguishable without the aid of a powerful microscope (Hurlbut and
Switzer 1979). Chalcedony consists of microscopic fibrous quartz
crystals with minute pore spaces that may be filled with water or air.
The pore spaces may also contain minute particles of other minerals that
produce that attractive coloring and banding in some agates (Sinkankas
1975). The granular type is made up of roughly equidimensional rather
than fibrous microcrystals and includes jasper, flint, and chert
(Hurlbut and Switzer 1979). The distinction between fibrous and granular
cryptocrystalline quartz is not universally recognized and the two
types are often lumped together as chalcedony (Schumann 1997).
 |
| Silicified ironstone concretions with distinct coarse botryoidal habit. Photo by Wayne Sutherland. |
The
most common granular varieties of cryptocrystalline quartz are flint
(dark-brown to almost black due to impurities) and chert (commonly
opaque light gray to white). The two grade into one another and both
have dull luster. Although these were used extensively by early man for
tools, today these have very limited use as a semi-precious gem. Jasper,
also granular, is typically deep red, reddish-brown to yellow-orange
due to the presence of traces of iron oxide within jasper. Jasper is
almost opaque and has a dull luster (Hurlbut and Switzer 1979) and is
commonly used as a low-value ornamental stone or lapidary semi-precious
stone.
Fibrous
cryptocrystalline chalcedony commonly forms botryoidal masses with
fibers oriented perpendicular to the hummocky surface. Chalcedony also
occurs as cavity and void fillings or linings in various rock types. It
is also found as fracture fillings and as replacements of organic
material such as petrified wood. Wide variations in color, banding and
patterns of inclusions are the source of the prolific names of
sub-varieties of chalcedony (Hurlbut and Switzer 1979).
Cleavage and fracture.
Pure chalcedony has no cleavage and breaks with uneven rough to
splintery, or conchoidal fracture similar to glass (Sinkankas 1959;
Bauer 1968; Schumann 1997).
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| Silicified and polished dinosaur bone from the Morrison Formation |
Specific gravity.
The specific gravity of chalcedony is 2.58 to 2.64. This is slightly
lower than coarsely crystalline quartz because of its slight porosity
(Sinkankas 1959; Hurlbut and Switzer 1979).
Size. Chalcedony can vary in size from small grains to large masses weighing tons.
Optical Properties
Color.
Pure chalcedony is pale blue to white to pale gray, with other colors
caused either by chemical impurities or by mineral inclusions (Sinkankas
1959; Schumann 1997).
Fluorescence/Piezoelectricity.
Chalcedony has no reported piezoelectric properties. Pure chalcedony is
fluorescent blue to white, whereas fluorescence in other varieties
ranges from nonexistent to strong yellow to blue-white depending on the
presence of chemical impurities or mineral inclusions (Schumann 1997).
Some of the popular Sweetwater moss agates from the Granite Mountains in
central Wyoming fluoresce brilliant yellow due to presence of hydrous
uranium arsenate (Hausel and Sutherland, 2000).
Index of Refraction, transparency, pleochroism.
The refractive index for cryptocrystalline quartz is about 1.53
(Hurlbut and Switzer, 1979). Sinkankas (1959) shows the refractive
indices for chalcedony at 1.533 and 1.539, and Schumann (1997) states a
range for pure chalcedony of 1.530 to 1.540, with a slight double
refraction up to 0.004 with no dichroism. Pure chalcedony is translucent
to transparent with a waxy to dull luster, but may vary to almost
opaque with mineral inclusions (Sinkankas, 1975; Schumann, 1997).
Chatoyancy. Chatoyancy is displayed in some varieties of chalcedony.
Adularescence.
Adularescence is rare in chalcedony. However, violet adularescent
chalcedony is described in specimens that were found in Iran by Douman
and Quinn (2004).
Varieties of Chalcedony
Agate.
Chalcedony nodules with banded colors (other than red, brown to yellow)
are typically referred to as agates. The color bands tend to parallel
the outsides of the nodules, or may form horizontal layers within the
nodules. These derive from their formation as cavity linings and
fillings in a variety of host rocks. The banding is usually distinct;
however, common usage applies the name agate to varieties of chalcedony
that also show no banding, such as moss agate (Sinkankas 1975). Thus,
agate generally refers to the lighter-colored chalcedonies, whether
banded or homogeneous.
 |
| Youngite specimen from the Hartville area. |
Many
descriptive or local names have been applied to agates (Sinkankas 1959
and 1975; Mason and Berry 1968; Schumann 1997). A short list of a few of
these includes:
- banded agate – an agate that exhibits distinct color banding.
- fortification agate – an agate with banding that flows outward into several points within a nodule to provide the appearance similar to a medieval fortress.
- eye agate – an agate with concentric banding surrounding a point in the center that gives the appearance of an eye.
- agate breccia – an agate formed of broken broken lithic fragments that is rehealed by chalcedony and or quartz.
- moss agate - a translucent chalcedony that encloses moss-like manganese or iron oxide dendrites.
- botryoidal agate – agate that exhibits botryoidal texture. An external hummocky to rounded form similar to bunches of grapes.
- dendritic agate - moss agate that exhibits a distinct dendritic pattern.
- flame agate - a dendritic agate with red to orange flame-shaped dendrites
- iris agate – an agate that exhibits a spectral display of colors due to microscopic diffraction grating caused by alternating bands of material that has higher and lower refractive indices.
 |
| Cut and polished Youngite agate breccia |
Bloodstone. Bloodstone is a green opaque chalcedony (or agate) with red spots. It is also known by the earlier Greek name, heliotrope (Mason and Berry 1968; Hurlbut and Klein 1977; Schumann 1997).
Carnelian.
Translucent red to orange-red and brownish-red chalcedony that is
colored by hematite is known as carnelian. Carnelian grades into sard and is similar to jasper, in fact; many specimens of carnelian can be cut from many pieces of jasper (see jasper).
According to Vanders and Kerr (1967), India is the principal source for
carnelian, but other sources include Wyoming, Washington, Colorado and
Michigan (Hausel, 2009).
 |
| A very attractive Montana blue agate sent to me by a collector from Montana. |
Onyx.
Onyx is made up of alternating dark and light colored straight parallel
bands or layers of chalcedony, usually black, white, red, and yellow. A
similar appearing onyx is distinguished from onyx chalcedony which is known onyx marble. Onyx marble is considerably softer and easily scratched (Hausel, 1986; 2009). Some excellent onyx marble is found in the northern Hartville uplift of Wyoming (Hausel and Sutherland, 2000).
Petrified wood.
Petrified wood, also known as fossilized wood is produced by
silica-rich solutions in groundwater replacing buried organic material.
The silica-rich solutions are supersaturated in silica and slowly
replace organic material in entire plants and trees with silica
producing pseudomorphs of the plant that often contains extraordinary
details of the original tree all the way down to cellular structure.
Cryptocrystalline quartz of many types, including agate and jasper, may
be found in petrified wood.
Petrified
wood is found on all continents with one of the most spectacular
examples being the Petrified Forest National Monument and surrounding
areas in northern Arizona where Triassic Shinarump and Chinle Formations
contain numerous petrified wood tree trucks scattered all over the
surface. Petrified wood is also known in the Eden Valley and Blue Forest
areas of southwestern Wyoming, from the Wiggins Fork area in Absaroka
Mountains of Wyoming, and from Yellowstone National Park in northwestern
Wyoming.
 |
| Tiger's Eye specimen - photo courtesy of Tom Nissen |
Tiger’s Eye.
This agate expresses chatoyancy as a golden yellow color on a brown
background. Depending on the background or base color, such agates
receive various gemological and rock hound terms. When the background
color is greenish-gray or green the gem may be known as cat’s eye quartz. When it is a blue-gray to blue, it is known as hawk’s eye quartz, and a stone with mahogany color base is called bull’s eye quartz. The chatoyancy is usually enhanced in rounded, polished, ornamental stones or cabochons.
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| Chalcedony colored by fuchsite mica from Copper Mountain, Wyoming |
Most
tiger’s eye comes from South Africa although it also occurs in lesser
deposits in California (USA), Australia, India, Myanmar, and Namibia
(Schumann 1997).
Geology & Genesis
Primary Deposits. Quartz
is a significant component of many igneous, metamorphic, and
sedimentary rocks, as well as quartz veins, quartzites, and sandstones.
The modes of formation for coarsely crystalline and cryptocrystalline
quartz differ in general, but overlap under low temperature conditions.
Coarsely crystalline quartz forms under both magmatic and hydrothermal
conditions, but may also form by precipitation from silica-saturated,
relatively low temperature hydrous fluids, which is the common mode for
the formation for chalcedony. In other cases, desilication of country
rocks followed by migration of silica in solution to dilational zones
during regional metamorphism is also apparent in places.
 |
| Jasper with fold from Tin Cup district, Granite Mountains, Wyoming |
Quartz
is one of the more common rock forming minerals and most quartz has
little value as a gemstone, but is considered more as a semi-precious,
lapidary, or ornamental stone. Some quartz, because of its crystal
habit, color, transparency, or mineral inclusions, may have value as a
semi-precious stone, or low-value secondary gem. The best quality
macrocrystalline quartz varieties are faceted or kept as specimens,
while lower-quality material is made into cabochons. Both coarsely
crystalline quartz and chalcedony have been carved, although their
brittleness generally negates the extremely delicate type of artwork
found in the much tougher jadeite and nephrite. Cryptocrystalline quartz
varieties are most often finished as cabochons. The translucent to
transparent character of chalcedony combined with its moderate hardness,
waxy luster, ability to take a polish, and its wide range of colors
make it popular as a low-cost gemstone.
Famous Gemstones & Specimens
Large
amethyst crystals and other quartz gemstones are not common; one of the
largest amethysts was 10 inches in diameter, and found in Thunder Bay
Canada. A similar size amethyst was found in Maraba, Brazil. A cut
amethyst in the possession of the British Museum of Natural History
weighs 343 carats, and a large Brazilian faceted stone of 1,362 carats
is located in the Smithsonian collection (Kievlenko 2003). Some
enormous, well-formed common quartz crystals weighing up to 289 pounds
have been found in the past (Mattana, Crespi and Liborio 1977) and an
extraordinary quartz prism found in Mina Geris, Brazil, weighed more
than 3,500 pounds (Anomalous 2003). Some of the more impressive,
transparent, colorless quartz including some very large specimens have
been recovered in Hot Springs County, Arkansas (US).
Very
large, well-formed translucent quartz and some large fluorite crystals
were found in the Holy Moses pocket on the Godsend claim located in the
Crystal Creek area near Lake George south of Denver, Colorado. The
pegmatitic minerals were recovered from a miarolitic cavity in the Pikes
Peak batholith. Some of the specimens included smoky and colorless
quartz weighing up to 439 pounds (Berry and Fretterd 2003).
 |
| Banded onyx agate enclosing geode with crystalline quartz. Specimen from the Jay Sundberg collection, Rawlins, WY |
Deposits
This discussion
of deposits here must be considered as only a sampling of a voluminous
subject due to the abundance of quartz-family gemstones and their
wide-spread occurrences. As such, the focus is on amethyst for coarsely
crystalline varieties, and on a few chalcedony occurrences, with
emphasis on those from Wyoming. This does not negate the importance of
other occurrences, merely a method of providing examples from a catalog
that would otherwise run into thousands of pages.
The
finest amethysts are produced from Brazil, Uruguay, the United States,
Madagascar, Russia, India, Australia and South Africa. Large quantities
of citrine are found in Brazil, the United States, Spain and Russia.
Rose quartz is mainly recovered from Brazil, the United States and
Madagascar. Cat’s eye quartz is produced in Sri Lanka, Burma and Germany
(Cipriani and Borelli 1986). South Africa has been an important source
for hawk’s eye, tiger’s eye and bull’s eye quartz.
Australia Some
amethyst occurs in the northeastern portion of Queensland, in eastern
New South Wales, and in the northeastern and central portions of
Victoria. The amethyst from the Mount Phillips deposit 360 km northeast
of Canarvon in Western Australia, occurs in a dike of
nontronite-tremolite within migmatites and biotite schist xenoliths. The
amethyst forms masses of druses weighing up to 50 kg, and some crystals
weigh up to 5 kg. The stones typically have a dark violet color with
small transparent zones that can be used for gemstones (Kievlenko 2003).
Popular specimens of gold in quartz have also been produced from
Australia (Laurs, 2005c).
Bolivia Amethyst
deposits in Bolivia are found in the Bolivian craton and are
structurally controlled in zones of silicification developed along
faults that cut Cambrian limestones. The better amethysts have dense,
violet color and are extracted with citrine, as well as with colorless
and smoky quartz crystals. A popular gem, known by the trade name
ametrine, is zoned, bicolor quartz consisting of a mixture of amethyst
and citrine. Some amethyst and quartz is also found in Proterozoic
crystalline rocks in breccias and veins (Kievlenko 2003).
Brazil. Brazil produces much of the world’s quartz gemstones and has done so since their discovery there in the 19th
century. Brazil leads the world in it production of amethyst and other
coarsely crystalline quartz varieties (Sinkankas, 1959; Bauer, 1968;
Gübelin and Erni, 2000) including crystals with unusual inclusions
(Hyrsl, 2004a). Brazil is also a major producer of agate, which often
occurs within the same deposits as coarsely crystalline quartz.
Hydrothermal
amethyst is hosted by Proterozoic quartzite in the southern part of the
State of Bahia in eastern Brazil. The fracture-filling veins include
distinct veins, stockworks and cavernous fracture zones that are noted
for their massive amethyst and euhedral crystals in cavities. Some veins
have been traced for several hundred meters along strike and may
exhibit widths of 2 to 3 m (6.4 to 9.6 feet).
The
Montezuma deposit is considered the largest of these. Obelisk and
prismatic amethyst with transparent caps occur in crystal form up to 20
cm in length (8 inches). The amethyst is found in the veins with
kaolinite-illite. At the Fazenda Serra do Salto deposit, amethyst is
related to a zone of fracturing in white to pink sericitic bearing
quartzites. Rod-like greenish gray quartz with amethyst may be as much
as 30 cm (12 inches) in length and may cement the breccia. Much of the
amethyst is apparently heat-treated to produce yellow and orange-brown
citrine under a trade name of ‘Rio Grande topaz’. At the Grogo do Choch
deposit, amethyst is associated with stockworks and solution cavities in
quartzite. The amethyst is found in radiating groups of crystals on the
walls of fractures and ranges from to 3 to 30 cm (1.2 to 12 inches) in
length, and is found in some geodes up to 1 m (3.2 feet) in diameter.
Some scepter-terminated amethyst is also found at Grogo do Choch. The
scepters sit on milky white to brownish red and translucent prisms.
Well-formed,
lustrous graphite cylinders, completely enclosed in quartz, have been
found in rock crystal from Bahia. In rare specimens, the graphite is
botryoidal. Doubly-terminated crystals up to 3 cm long are common, with
some as much as 10 cm. Where the graphite extends to the crystal
surface, it has either been replaced by yellowish-brown iron oxides or
removed completely, with only a hollow cavity remaining (Hyrsl, 2004b).
 |
| This is not really a photo of some quartz, but instead it is a strategically placed ad for one of my books. |
Exceptionally
fine examples of rutilated quartz have been extracted from the Golconda
mine in Minas Gerais (Koivula and Tannous, 2004e). Brazil has been
known to have produced rutilated quartz with colors ranging from the
more common golden-yellow to coppery-red and silver-gray (Koivula and
Tannous, 2004a).
Within
the Parana River basin, abundant large amethyst geodes occur in
tholeiitic basalts and basaltic andesites of the 40-50 m thick Lower
Cretaceous Serra Geral Formation in the Ametista do Sul region in the
state of Rio Grande do Sul. The volcanics occupy an area of about 1
million km2 and are as much as 800 m (2,500 feet) thick in
some areas. Large portion of the basalts are mineralized over
thicknesses of 2 to 10 m (6.4 to 32 feet). The amethyst, along with gray
and colorless quartz, agate, celadonite, calcite and occasional barite,
fills flat-lying joints in the amygdaloidal basalts and also cements
local fracture zones. The locality in southern Brazil is well-known as a
source of Palmeira amethyst (Schumann, 1997), including both faceting
material and geodes. The color is lilac with a bluish tint and will turn
brownish orange following heat treatment at temperatures of 435 to 475oC (Kievlenko 2003).
 |
| Discovery of a giant jasper deposit, Rattlesnake Hills, Wyoming. Jasper, jasper, jasper everywhere on Jasper Hill in the Rattlesnake Hills gold district. Photo shows my field assistant - Wayne Sutherland (photo by the author). |
At
the important Maraba deposit in the western portion of the State of
Para, 270 miles south of Belem, fractures in Proterozoic quartzites are
filled with light-violet amethyst aggregates (Schumann, 1997; Kievlenko
2003).
Canada Quartz-rich
breccia zones along recurrent faults within an unconformity between
Archean (2.7 Ga) and Proterozoic (2.0 Ga) rocks in the Thunder Bay –
Lake Nipigon area of Canada are local exploration targets for amethyst.
This 125 km long by 40 km wide amethyst region along the north shore of
Lake Superior was the prehistoric source for amethyst used by native
people. Modern mining there began in the 1860’s, but diminished in the
early 1900’s with competition from material mined in Brazil. Renewed
activity came in the 1950’s with the discovery of several large amethyst
deposits.
Amethyst
occurrences in the Thunder Bay amethyst region are related to the
hydrothermal vein system responsible for the Dorion lead-zinc-barite
veins and Thunder Bay silver. Vugs and zoned veins within this
environment host amethyst ranging in color from almost black to very
pale-violet. Market strength for as many as 14 producers, many of whom
offer fee-digging, is rooted in spectacular amethyst specimens often
found in combination with barite and fluorite, as well as in fine
facetable material (Garland, 2004).
 |
| Green fuchsitic quartzite from the Snowy Range, Wyoming |
This
amethyst field which includes the Thunder Bay, Diamond Willow, Dorion
and Ontario gem mines lies along the northwestern coast of Lake Superior
near Thunder Bay, Ontario. The amethyst is in barite-quartz veins that
are up to 1.5 m (4.8 feet) thick. The veins occur in Late Archean
granite gneiss, granite and quartz monzonite near the contact with
Proterozoic sedimentary rocks. Veins, lenses and 15 to 25 m (48 to 80
feet) thick mineralized fractures are grouped in vein-stockwork zones
that trend for 1.6 km (Thunder Bay) and 2.4 km (1-1.4 miles) (Diamond
Willow). The amethyst was deposited on the walls of narrow fractures as
radiating groups of fine crystals, compact aggregates, and large
pyramidal shaped crystals up to 25 cm (10 in) in diameter that are found
in clay-filled cavities in vein swells. These cavities may contain
druse-like crystal aggregates in masses up to 300 kg.
The
color of the amethyst ranges form pale lavender to dense violet. The
amethyst is interpreted to have deposited in iron-rich solutions that
were depleted in aluminum under oxidizing conditions at 90 to 250oC and low pressures. The amethyst mineralizing event was preceded by quartz-barite-sulfide mineralization (Kievlenko 2003).
Chile Transparent
rock crystal with phantoms formed by thin, platy, lead-gray metallic
molybdenite inclusions have been recovered from the Confianza mine,
Tilama, Valparaiso. These well-formed, singly- and doubly-terminated
crystals are reported to have weights in excess of 10 kg (Koivula and
Tannous, 2004d).
China Hydrocarbon inclusions within quartz are abundant in Sichuan Province, China (Hyrsl, 2004).
Europe Amethyst is mined in the Rhodope Mountains of Bulgaria (Balitsky, Balitsky, Bondarenko, and Balitskaya, 2004).
Iran Violet
adularescent chalcedony has been mined from an area underlain by Eocene
andesitic volcanic rocks near Qom Salt Lake in central Iran since 2003.
The material is recovered as glauconite encased, irregular nodules that
exhibit botryoidal structure and some agate-like color banding. The
best adularescent chalcedony varies from dark-violet to light
grayish-violet, and is accompanied by agate in colors of brown, orange,
yellow, and pink. Production was reported in 2004 at 800 kg of
chalcedony per month, with only about 150 kg exhibiting moderate tones
of violet color. The translucent violet adularescent chalcedony, having a
specific gravity of 2.59 and refractive index if 1.538, is processed
into cabochons weighing up to 22.5 carats (Douman and Quinn, 2004).
Madagascar Colorless
rock crystal quartz from the Sakavalana pegmatite, Fianarantsoa
Province hosted a pink-zoned inclusion of pezzottaite (a Cs, Li-rich
member of the beryl group) (Koivula and Tannous, 2005).
Chatoyancy
resulting from coppery-red rutile within highly transparent quartz is
one of many types of quartz that has come from Madagascar (Koivula and
Tannous, 2004a).
Namibia Amethyst
and citrine are produced from the Sarusas mine in a remote part of
Skeleton Coast Park in the Namibia Desert of northwestern Namibia.
Amethyst and rare pale citrine occur as geodes within basalt. Quartz
mined here during the late 1960’s through the mid-1980’s was entirely
shipped to Germany. The mine was reopened in late 2004, producing about
1500 kg of rough from which less than 1% was facetable. Cut stones
weighed up 20 carats, with amethyst colored light to dark purple, and
citrine an orange-yellow to deep orange. Commercial citrine is produced
through heat treatment of amethyst at as much as 600˚C (Laurs, 2005b).
Pakistan Quartz
crystals hosting both fluid and solid natural petroleum inclusions have
been mined in Baluchistan (Koivula and Tannous, 2004b).
Russia Amethyst
deposits are found in the Ural Mountains and Yakutia. One hundred
kilometers north of Ekaterinburg in the Murzinka-Adui region of the
Central Ural Mountains, a large amethyst deposit is found along on a
contact between Late Paleozoic Murzinka granite with Cambrian
biotite-amphibolite gneiss. Topaz- and beryl-bearing miarolitic
pegmatites occur in gneiss, while hydrothermal amethyst is mainly in
granite west of the Murzinka and Sizikova villages.
The
principal amethyst deposit is the Vatikha in the Sizikovsky district.
The district itself includes closely spaced quartz veins and mineralized
fractures in granite. The quartz in the stockworks is associated with
several faults that are mineralized along a 30 to 350 m (96 to 1120
feet) strike length. The veins range from 5 cm to 1 m (2 inch to 3.2
feet) thick and mineralization continues to depths of at least 150 m
(480 feet) based on mining and drilling. The veins and stockworks are
accompanied by intensely propylitized and argillized selvages that
produce distinctly greenish altered granite. The walls of the fractures
are coated with gray translucent quartz and the open veins contain some
colorless and smoky quartz, amethyst and carbonates. The vein mineralogy
also includes hematite, siderite, brookite, analcime along with
secondary goethite, psilomelane and gypsum. Altered propylitized wall
rock includes secondary kaolinite, sericite, chlorite, albite,
carbonates, pyrite and quartz.
Another
group of amethyst deposits are found 80 km (48 miles) southwest of
Aldan in southern Yakutia. The amethyst is found in syenite porphyries
at their contact with granites. Where found, the amethyst occurs in
hydrothermally altered granite and syenite porphyries that contain
numerous fractures and quartz lenses associated with sericitization,
kaolinization, and silicification. The main amethyst zone occurs in a
fracture zone in syenite that was traced for 165 m (528 feet) along
strike. The amethyst is found in cavities in swells and at the
intersections of veins and fractures. The cavities reach up to 3 m (9.8
feet) in length with cavity walls covered by druses of smoky quartz and
crystals of amethyst that are 1 to 8 cm long (0.4 to 2.4 inches). These
occur primarily unattached to the walls, lying in the clay-sand material
filling the cavities (Kievlenko 2003).
Tajikistan The
Selbur deposit in Tajikistan forms a large deposit of quartz with
amethyst about 45 km (27 miles) west of Dushanbe in the Hissar
Mountains. The deposit is part of a Middle-Upper Carboniferous
volcano-sedimentary succession consisting of tuffaceous sandstones,
siltstones, and limestones. The deposit consists of greenish-gray
polymictic feldspar-quartz sandstones with sericite and chlorite that is
cut by northerly trending fractures. These fractures zones exhibit
silicification, potassic feldspathization, and locally some
carbonatization and ferrugination, and contain numerous conjugate
quartz-amethyst veins and lenses including a group of mineralized zones
that are as much as 50 to 350 m (160 to 1120 feet) long and 1 to 8 m
(3.2 to 25 feet) thick.
The
veins and lenses are essentially quartz veins with some ankerite and
calcite. The quartz is primarily milky with gray comb-like translucent
quartz and amethyst in the central portions of the veins. Cavities are
periodically found in the central portions of the veins, with radiating
quartz and amethyst crystals – some of the larger cavities also contain
orange-red clay. Amethyst crystals have prismatic habit and typically do
not exceed 5 cm (2 inches) in length. These for the most part are
cloudy and fractured, and not suitable for gems. The gem material occurs
in the comb-like portions of the vein. The amethyst ranges from pale to
intense violet with a smoky to reddish tint.
Near
fracture zones, the sandstone is bleached and silicified and trapped
xenoliths are intensively argillized. Other veins lacking in amethyst
contain fine-grained sugary quartz and lack in alteration selvages
(Kievlenko 2003).
United States
According
to Kievlenko (2003) there are a number of small hydrothermal amethyst
deposits in the US that are mainly found in the Appalachian Mountains in
Virginia, North Carolina, South Carolina, and Maine. Other amethyst
deposits are reported in Arizona and Montana.
Arizona High-quality
deep purple amethyst is produced from crystal-filled or crystal-lined
cavities in the Four Peaks mine near the south end of the Mazatzal
Mountains in Maricopa County, about 75 km (46 miles) northeast of
Phoenix. Smoky and transparent quartz are produced here as well. This
deposit, considered by some to be the most important commercial
gem-quality amethyst source in the United States, has been mined
intermittently since the early 1900’s. Rough crystals may be as large as
20 cm (8 inches) in diameter, but most are only clear enough near their
tips to supply gems (Sinkankas, 1959). Cut stones have exceeded 20
carats in size, and show a wide range of purple colors including a very
dark-purple and reddish-purple. Uneven color zoning with banding
parallel to rhombohedral crystal faces, crystal faces coated or frosted
with hematite and apatite, and variable transparencies in rough crystals
are characteristic of the deposit. Finished stones may host hematite
flakes and fluid inclusions, and may show growth zoning and Brazil-law
twinning, all of which distinguish amethyst from the Four Peaks mine
from synthetic material. Heat treatment is used on 20-30% of Four Peaks
amethyst to lighten its overly-dark color, although subsequent
fracturing renders about 50% of the treated material useless (Lowell and
Koivula, 2004).
 |
| The Superstition Mountains in Arizona - home of the Lost Dutchman Gold Legend. |
Most
recently, the mine on a 20-acre patented mining claim within the Four
Peaks Wilderness Area and at an elevation of 1981 m (6500 ft), has been
owned and operated by Four Peaks Mining Company from Ocean Grove, New
Jersey. The mine workings are limited to an open cut and a 10 m tunnel.
Mining is accomplished using pneumatic and hand tools (Lowell and
Koivula, 2004).
The
Four Peaks represent Precambrian metasediments that were intruded by
Proterozoic granite. Development of linear zones of brecciation within
the light-colored Mazatzal quartzite accompanied granite emplacement.
Irregular cavities, generally 0.3 to 1.2 m (1- to 4-ft) deep and up to 3
m (10 ft) long, within the brecciated quartzite host amethyst as well
as rock crystal and smoky quartz. These crystals are thought to
originate from silica-rich hydrothermal fluids derived from the cooling
of the intrusive granite (Lowell and Koivula, 2004). Cavities are often
clay-filled, with quartz crystals either attached to the walls or loose
within the clay. Detached crystals commonly show signs of extensive
corrosion, probably related to late stages of cavity development.
Apatite and hematite are common accessory minerals, and minute
fluorapatite and hematite crystals are found as coatings on corroded and
etched quartz (Sinkankas, 1959).
Arkansas
- Nearly flawless prismatic quartz is recovered from several deposits
in Hot Springs County. The material exhibits excellent formed,
transparent prisms terminated at one end by a pyramid.
California - Striking
specimens of gold-in-quartz have been produced from the Badger mine,
Mariposa County, California. The mine was first worked in the 1850’s,
producing $80,000 in gold from shallow tunnels and trenches in the
Prescott quartz vein system. The vein is up to 10 m wide, and is
prominent throughout the mining area. Gold-in-quartz is also produced
from the Sixteen-to-One mine in Sierra County (Laurs, 2005c).
Amethyst
has been known for many years from the Kingston Range in San Bernardino
County, California. Notable gem-quality material was recently extracted
from the Purple Lily prospect, including crystals as much as 9 cm (3.5
in) in length. The amethyst occurs in cavities along northwest-trending
fault zones and joints within granitic host rock. Amethyst crystals also
are found within miarolitic cavities in areas of light-colored granite
(Laurs, 2004).
Naturally occurring green quartz or prasiolite is found on the California-Nevada border (Quinn, 2004).
 |
| Quartz crystal in nephrite (jade) from the Granite Mountains, Wyoming. |
Georgia - Amethyst
crystals have been surface mined since the mid-1980’s from weathered
granitic host rock near Jackson’s Crossroads in Wilks County, Georgia,
about 150 km (93 miles) east of Atlanta. Recent mining, beginning in
2004, by Terry Ledford and Rodney Moore has produced cut stones larger
than 48 carats that display a deep purple color in incandescent light
and a noticeably more blue color in daylight. The site is open to public
fee-digging (Laurs, 2005a).
Montana
- Amethyst is found in the Toll Mountain pegmatite in the southwest
portion of Jefferson County near Butte. The miarolitic pegmatites are
found in Mesozoic granite of the Boulder Batholith and are concentrated
near granodiorite and quartz monzonite facies in the batholith. The
principal deposits, referred to as the Pohndorf and Little Gem mines,
were developed on three stock-like pegmatites. Amethyst is also reported
in the East Range, Timber Butte, Lake Delmo and Goldflint Mountain
areas (Kievlenko 2003).
New York - Doubly-terminated
quartz crystals from Herkimer County, New York have been known to host
amorphous hydrocarbons as inclusions, referred to as “asphaltite” or
“anthraxolite”. These originate within silicified dolomites, and appear
to have developed during digenesis of the host rock (Hyrsl, 2004).
Hydrocarbon inclusions are also known from other localities around the
World, however, those from Herkimer are the best known. The small
colorless quartz crystals from this deposit are widely known as
‘Herkimer diamonds’ (Hurlbut and Switzer, 1979).
North Carolina - Coppery-red rutilated quartz has been mined from North Carolina (Koivula and Tannous, 2004a).
Wyoming - Many
varieties of quartz and chalcedony have been found in Wyoming. Good
specimens of prismatic quartz are uncommon in Wyoming, but the state has
yielded many attractive specimens of chalcedony. Much of the
crystalline quartz found in the state has been cloudy and translucent to
opaque. However, specimens of chalcedony are quite varied, and Wyoming
has become a favorite collecting ground for agate, jasper, flint, and
petrified wood.
 |
| Malachite (green), azurite (blue) and limonite (tawny). |
Many
different agates are found near Guernsey in the Hartville uplift of
eastern Wyoming. These have attracted rock hounds and mineral collectors
for years, and many varieties of agate have received local names. For
example, one local agate known as Youngite,
forms distinct rehealed limestone breccias consisting of pink- to
cream-colored breccia clasts of Guernsey Limestone that are cemented by
light-grey to greyish blue, banded, drusy quartz and chalcedony (Figure
3.43). The drusy quartz fills fractures in the breccia and completely
encases the brecciated limestone clasts. When polished, the contrasting
colors of the quartz and breccia clasts provide very attractive lapidary
stones. Under long wavelength fluorescent light, these samples yield a
blue network of fluorescence that follows the bands of drusy
quartz. Samples of Youngite are of relatively high-value.
Another popular agate found in the same region is the Slater agate. Slater
agates form concretionary masses with white coatings. When cut, the
interior of the stones are typically dark grey to black agate
surrounding milky, agatized, interiors with small, fine, agatized
dendrites. The surface of some of these agates is porous and may give an
impression of a fossilized sponge.
Petrified
wood, quartz, and chalcedony are found in the Laramie, Sierra Madre and
Seminoe Mountains, and in the Shirley, Laramie, Goshen Hole, and
Saratoga Valleys in southeastern Wyoming. The most common forms of
chalcedony in this region are agates and jaspers eroded from Paleozoic
limestones, particularly, the Casper Formation. Some petrified woods
from this region are probably derived from silica leached from Tertiary
volcanic ash falls, that later replaced the wood.
In
the Saratoga Valley, agatized and opalized woods and dendritic agates
are found on the flats north of town. The host rock was possibly
tuffaceous sandstone, siltstone, and claystone of the North Park
Formation.
Beautiful
specimens of jasperized iron formation are found in the Seminoe
Mountains several miles northwest of Saratoga, and north of
Sinclair. These rocks are magnetic and have alternating bands of black
magnetite, dark gray quartz, and tawny to brown layers of jasper and
grunerite. The source of this material is banded iron formation from
Bradley Peak in the Seminoe Mountains gold district. Much of the
jasperized material is found as cobbles and pebbles downstream along
Deeweese Creek and in alluvium along the north flank of the range near
the Miracle Mile on the North Platte River. This area should also be of
interest to the gold prospector. The Miracle Mile is surrounded by a dry
placer formed of unconsolidated gravels located several feet above the
river banks. The authors have recovered gold colors from badger and
other burrowing animal diggings in this region. They have also found
diamond stability pyrope garnets associated with pediment gravels in the
same area.
At
Shirley Basin east of the Seminoe Mountains, several agates and jaspers
have originated from Casper Formation limestones and sandstones. One
agate is a reddish-brown jasper mixed with dark gray to black opaque
agatized breccia near Mine Hills and Crystal Hill to the south of
Marshall. This rock consists of pink breccia clasts of Casper Formation
sandstone cemented with reddish-brown and massive black chert coated
with tiny grains of psilomelane (metallic, manganese oxide) that
produces an attractive decorative stone. Along the southern end of the
basin, the Shirley uplift is famous for silicified dinosaur bones found
on both sides of Como Bluff north of US Highway 287/30 near Medicine Bow
(Sinkankas, 1959).
 |
| Quartz from Big Creek district, Wyoming |
The
Absaroka Mountains in northwestern Wyoming, have produced a variety of
petrified woods and agates. Along the western margin of the range,
Yellowstone Park is famous for its fossil forests at Specimen Ridge and
Amethyst Mountain. Some prostrate fossilized trunks have been found in
this region that are more than 50 feet long, and 5 feet in diameter
(Sinkankas, 1959). Amethyst is also reported in the Yellowstone area at
Amethyst Mountain (keep in mind, collecting is prohibited in the
National Park).
 |
| Jasper breccia from Tin Cup district, Granite Mountains, Wyoming |
East
of the Absaroka Mountains, chert, agate, and jasper occur in Paleozoic
limestones along the eastern flank of the Bighorn Basin, along the
southern margin of the Bighorn Mountains, and the north side of the Owl
Creek Mountains. Woodcast agates are reported along the Bighorn River
and in several drainages on the western margin of the basin. Green
agates and crystal-lined geodes have also been reported along Cottonwood
Creek near Hamilton Dome.
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| Here is an extremely rare geode! This is actually a peridotite nodule collected from a kimberlite in Wyoming - note the two large pyrope garnets on the side. The center of this peridotite was dissolved and coated with tiny prismatic quartz crystals. |
In
the Black Hills of northeastern Wyoming, some chalcedony and amethyst
is found. Agates and jasperoids are also reported in stream gravels
along with gold colors and nuggets in the Mineral Hill district of the
Black Buttes east of Sundance. These appear to be related to Tertiary
alkalic volcanic rocks.
In
the Powder River Basin, west and south of the Black Hills, several
varieties of petrified wood, chert and jasper are found. The chert and
jasper are associated with Paleozoic limestones along the western margin
of the basin, and the petrified wood is derived from the Wasatch
Formation. Occasionally, some spectacular, large-diameter stumps and
logs are found. East of Buffalo at the Dry Creek Petrified Tree site,
poorly silicified, brittle, wood is found that easily crumbles into
small pieces. The petrified wood in this area is primarily of scientific
interest and is not suitable for polishing.
Another
petrified wood, the Crazy Woman Creek wood, is more durable and suited
for lapidary. This wood is silicified, banded in shades of brown and
white, and found in terrace gravels about 60 to 120 feet above Crazy
Woman Creek in an area extending from the Bighorn Mountains to where Dry
Creek joins Crazy Woman Creek. Large pieces (longer than 1 foot in
length) of the Crazy Woman petrified wood were found in the vicinity of
Crazy Woman Creek in the past, and can still be found in landscaping and
local collections in Buffalo. Similar material has been found in
terrace gravels along the Powder River near Kaycee. East of Buffalo,
amethyst-lined cavities were reported in some specimens of petrified
wood.
 |
| Yep, another strategically placed book ad. |
The
Granite Mountains in central Wyoming are known for jade. In addition,
several types of agate, jasper, and petrified wood and some rubies and
sapphires have been found. One of the better-known varieties of agate is
the Sweetwater moss agate which usually occurs as small pebbles in lag
gravel and in the basal conglomeratic sandstone of the Split Rock
Formation. These agates fluoresce brilliant yellow due to the presence
of hydrous uranium arsenate. They often contain a brown, opaque surface
that can be removed by tumbling. Tumbling results in a highly polished,
light gray to blue agate with black manganese dendrites.
In
the same general region, the Ice Point Conglomerate, has rounded
fragments of black petrified wood, and the Bridger Formation contains
fossil tree stumps, fragments of petrified wood, and dark gray and brown
agates (Love, 1970). Some clear chalcedony balls have also been found,
that are coated with opal and white ash. These have loosely been termed
'moonstones'. Another popular agate, the 'Angel' agate, is found in a 6
inch zone in the upper porous sandstone of the Split Rock Formation. The
agate occurs in nodules and is an attractive pale-greenish-gray color
with a chalky-white surface coating. The agates fluoresce a brilliant
greenish-yellow under ultraviolet light, and are slightly radioactive
(Love, 1970).
In
the nearby Tin Cup district, beautiful jasperized breccias with angular
fragments of blood-red, chocolate-brown, and butterscotch yellow-brown
jasper occur along three prominent faults which were prospected for gold
in past years. Some attractive chalcedony in this area is gray, banded,
folded agate.
In
the Eden Valley in southwestern Wyoming, petrified wood is found over a
wide area around Farson. This wood resembles ordinary weathered wood
and has an opaque cream colored coating of silica covering a silicified
black to brown core. The Laney Shale Member of the Green River Formation
and the overlying Bridger Formation appear to be the source of this
material.
Locally
the Bridger Formation contains petrified wood near Oregon Buttes. This
wood, known as the Bridger-type, consists of partially silicified black
wood. Where it is completely replaced by silica, it ranges in color from
brown, tan to green. Some clear chalcedony and vein moss agates are
found nearby, as well as paleoplacer gold.
Some
of the more striking wood found in Wyoming is Blue Forest agate west of
Farson in the Eden Valley. The wood has a black to brown central core
surrounded by clear blue chalcedony which producing an unique and very
attractive, silicified wood.
Reefs
and beds of silicified gastropods (fossil snails), which were deposited
in a prehistoric lake, are found throughout the area. Some of the
better collecting localities occur along Delany Rim south of Interstate
80 near Red Desert west of Rawlins, and to the west of Rock
Springs. Some fossil snails are light-brown in color and have a
weathered appearance, and will not polish. The darker agates found in
shades of dark brown to black, will generally polish, and produce
attractive bookends.
Banded
jasperoids on Quaking Asp Mountain south of Rock Springs consist of
dark- to light-gray banded agate with cross-cutting veins of quartz, and
banded red, yellow-orange, and gray jasperoid and onyx. These occur in a
very large and extensive silicified zone associated with ancient hot
springs. Some of these will produce beautiful lapidary stones (Hausel and Sutherland, 1998).
Uruguay Uruguay produces amethyst and other quartz gems, including agate, from deposits similar to those found in its neighbor, Brazil.
Zambia Hydrothermal
amethyst mineralization in Zambia is related to boundary faults that
separate basement Mesoproterozoic gneisses and metasediments that are
intruded by Late Proterozoic granitoids from Mississippian-Late Triassic
Karoo rocks. Amethyst deposits are contained in veins and stockworks
along these faults that occur within the basement complex at many places
in Zambia. The most important production comes from the Southern
Province in the Mwakambiko Hills and in the Mumbwa-Namwala area,
adjacent to Lake Kariba near the boundary with Zimbabwe (Kievlenko,
2003). Amethyst production from Zambia amounts to about 700 tonnes per
year, with the largest amount coming from Mwakambiko. The Mwakambiko
deposit is hosted in granosyenite that has intruded hornblende gneiss,
quartzites, and marbles. Abundant quartz-amethyst veins and lenses up to
1 m (3.3 feet) thick cut the brecciated, silicified, and
hematite-enriched host rock in a northerly trend for 3.5 km (2.2 mi).
Amethyst from the Mwakambiko deposit is extracted primarily from eluvial
debris Kievlenko (2003).
References
Hausel, W.D., and Sutherland, W.M., 2006, World Gemstones: Geology, Mineralogy, Gemology & Exploration: WSGS Mineral Rept MR06-1, 363 p.
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