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Chemical composition -- Silicon dioxide.
Quartz is one of the most abundant minerals on
Earth and is found in many different forms almost everywhere.
There are two major classifications of quartz: crystalline quartz
and cryptocrystalline quartz.
Color -- Very wide color range -- colorless, yellow, brown,
purple, pink, greenish. Cryptocrystalline quartz often displays exotic
color bands, swirls, and other patterns.
Optics -- R.I. 1.553-1.554.
Durability -- Hardness 7.
Crystal structure -- Hexagonal.
Specific Gravity -- 2.651 for crystalline
material, up to 2.91 for cryptocrystalline.
Sources -- Extremely widespread.
Crystalline quartz is quartz that occurs in distinct
crystals. It occurs in a number of familiar varieties distinguished
Amethyst is purple
quartz and is one of the most popular gemstones. It varies from pale lavender
("Rose-de-France" amethyst) to deep purple with red highlights
("Siberian" amethyst). Amethyst is generally abundant and quite
inexpensive (pennies to a few dollars per carat), although fine, large
Siberian amethyst is rather scarce and may command prices of several tens of
dollars per carat. When exposed to strong sunlight for extended periods,
amethyst may fade in color. Recently, gem markets have been flooded with
inexpensive synthetic amethyst. Although such synthetics can usually be
identified by testing for twinning, routine testing is not cost-effective,
and some new synthetics do not show the twinning effect.
- Citrine is yellow to
brown quartz. It varies from pale yellow to rich golden yellow
to dark orange. Although citrine may occur naturally, much is
produced by heating amethyst under controlled conditions (overheating
drives off all color, leaving colorless rock crystal).
Darker colors are more highly prized, including medium golden orange
("Rio Grande" citrine) and dark sherry-colored
- Ametrine is a combination
of amethyst and citrine
within a single crystal and was discovered only a few years
ago in Bolivia. Such bicolored stones are fairly unusual and can
be very striking in appearance, but prices remain rather low.
For a brief period, these stones were erroneously thought to be
manmade or treated to produce the bicolor effect, but it is now
known to occur naturally.
- Vermarine, also known
as Prasiolite or "Greened" Amethyst
is a light to medium green quartz produced by careful heating
of amethyst from one Brazilian location. Although inexpensive, it is seldom
seen and remains something of a collector's item.
- Rock Crystal is
clear colorless quartz. Its value tends to be very low, except
for large flawless pieces, which are rare. One of the finest known
pieces is the 12.75 inch diameter, 107 pound flawless sphere ("crystal
ball") in the collection of the Smithsonian Institution.
- Rose Quartz is pink
quartz. It is usually translucent to semitransparent. Strong rose-red,
transparent material is rare. Like amethyst, it may fade on prolonged
exposure to strong sunlight.
Smoky Quartz is tan or brown quartz. Very dark
reddish brown smoky quartz is known as cairngorm (from the Cairngorm
Mountains in Scotland), and black or blackish brown material is
known as morion.
Cryptocrystalline quartz is quartz in which the
crystals are microscopic in size and thus is always opaque or
translucent. It occurs in a huge array of colors and patterns
and, like crystalline quartz, occurs in myriad locations.
Agate is a chalcedony that displays an incredible
variety of color patterns -- generally curved bands of regular
or irregular formation. It occurs in myriad locations, and some
forms are rare and relatively expensive.
Aventurine is a more or less colorless chalcedony
that contains uniformly dispersed flakes of greenish mica, thus
giving the stone a characteristic speckled green appearance known
Bloodstone, also known as heliotrope,
is a dark green chalcedony or jasper with flecks of red.
Carnelian or cornelian is a reddish brown chalcedony. In ancient
Rome, it was often used in cameos and intaglios.
Chalcedony is a translucent or semitranslucent
cryptocrystalline quartz, which
may be patterned (agate) or uniform in
color (blue, green, pink, black, white, etc.).
Chrysocolla quartz is chalcedony mixed with varying amounts
of chrysocolla. The chrysocolla
provides the robin's egg blue coloration, and the quartz provides
sufficient hardness for use in jewelry. The durability varies
with the corresponding proportions of the two components.
Chrysoprase is a light to medium, slightly
yellowish green chalcedony. Australia provides many of the finest
Fire Agate is chalcedony
that contains many layers of tiny inclusions of limonite or goethite,
which produce a distinctive firelike iridescence when properly
cut to leave only thin protective layers of chalcedony over the
inclusions. Many of the finest specimens have been found in Arizona.
Jasper, in contrast to
chalcedony, is an opaque and more coarsely grained
cryptocrystalline quartz. Like chalcedony, it may be patterned or uniform in
Picture jasper, or scenic jasper,
may display quite realistic depictions of natural scenes, animals,
or other objects, and can command respectable prices due to such
Onyx is something of a catchall term that usually refers to
dyed black chalcedony ("black onyx"), but it is also used to describe
other colors of dyed chalcedony. More strictly speaking, onyx agate is
banded agate displaying straight and parallel bands of
alternating colors, which has historically been used to produce
cameos. (See sardonyx.)
True onyx, however, is a form of calcite, usually used
in architecture and ornamental pieces.
Petrified Dinosaur Bone or "dinny bone" is the
product of millenia-old fossilized bone (from dinosaurs!) in which the
cellular contents have been replaced by quartz, leaving the cellular bone
structure intact. The deserts of the American southwest (notably Utah) produce
many fine specimens.
Petrified Wood, like petrified bone, retains
the cellular structure of the original wood, but the cellular
contents have been replaced by quartz.
Prase is a light yellow-gray-green
Sard is a darkish brown
chalcedony similar to carnelian but
of less intense color.
Sardonyx is an agate
with alternating straight, parallel bands of reddish brown and,
Tigereye is a yellowish
brown cryptocrystalline quartz in which quartz has replaced crocidolite,
a fibrous, asbestos-like mineral. The parallel fibrous structure
produces a characteristic chatoyance (silky sheen), or even a
cat's-eye effect. Gray, green, and blue (hawk's-eye) tigereye
are also used as gemstones; they are usually the result of chemical
or heat treatment. Tigereye is often used for cameos
Turritella is an agglomeration of fossilized
turritella snail shells replaced by silica. Wyoming is the principal source.
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