Quartz is a crystalline rock or mineral composed of silicon dioxide. It is actually very common,
and occurs in many varieties, a number of which are gemstones, some of which are very beautiful
and very rare.
Quartz is generally divided into two groups: the transparent crystalline varieties and the
translucent to opaque chalcedonies that are microcrystalline forms of quartz. As noted above,
there are many different varieties in these two types but they have one thing in common: they
are very low in cost. Quartz is one of the most abundant minerals in the earth's crust. It is
also one of the most interesting.
Amethyst is the queen of the quartz varieties and in better qualities it is the most
expensive of the quartzes. Amethyst ranges from pale to dark violet. The finest qualities of
Amethyst are a medium dark violet with a strong secondary red color. Darker shades of Amethyst
may appear slightly differently under different light sources.
Amethyst of all qualities is available in all sizes and shapes. Light Amethyst will be very
low in per carat prices with the finest qualities still well under $l00 per carat. Darker
Amethyst of any size will have color zoning. This may not be visible without extensive
examination. Often the zoning consists of areas of red violet and areas of blue violet, this
combination can enrich the appearance of the stone. Sometimes the zoning is easily visible and
detracts from the beauty of the stone. If the zoning is unsightly the value is lessened.
Ametrine (pictured above, left) is a bi-color variety that is part Amethyst colored and part
Citrine colored. It was first discovered in Bolivia. Generally, Ametrine is cut into octagon or
emerald cut shapes to emphasize the two colors. Ametrine has been carved into some very
interesting gem art works. Prices will start in the low end and go to moderate price per carat
for exceptional pieces.
Smokey Quartz (pictured above, right) is brown transparent quartz of all shades, often called
smokey topaz; it generally has a slightly grayish cast. It is much easier to find Smoky Quartz
in large sizes; it is not always economically feasible to cut it into small sizes. Prices
should always be very inexpensive.
Colorless Quartz (also called "rock crystal") simply lacks interest as a jewelry stone. Even
when it is well cut it lacks brilliance. Occasionally it will contain some interesting
inclusions that can increase its appeal. Colorless quartz is occasionally found with rutile
needles inside which look like long golden threads, sometimes arranged in a pattern. This is
called "Rutilated Quartz" and is popular in art jewelry. Black Tourmaline crystals can also
create a pattern inside colorless quartz for a unique appearing gemstone. Other mineral
inclusions are possible. In rare cases, quartz will contain gold. Most gold bearing quartz is
milky white rather than transparent. These included quartz materials are being used for
carvings and can be gemstone art works.
Beads made from quartz have been popular for a hundred or so years; these are often faceted
into appealing shapes. Again these items are low in price. If a gem is carved into an "art
work" the quality of the work may be what you are paying for rather than the raw material;
these items will have to be valued accordingly.
Tiger's eye is quartz with asbestos inclusions; this gives it a subtle silky appearance.
Generally it is golden to brown in color; it can also be dyed. Blue material is called
Crocidolite. Quartz also comes in cat's-eye form; it is usually a golden yellow.
Star quartz is a fascinating stone; the quartz may be colorless or pink. The stars are
six-rayed and roll around the stone as it is moved. Sometimes Star Quartz is cut into spheres
which are extremely interesting.
Quartz Gemstone Groups
As noted, there are a number of different groups of quartz gems, differentiated by their exact
composition and crystalline structure. These groups are:
Fully Crystalline Quartz
This group includes rock crystal, amethyst, citrine, cairngorm and smoky quartz.
Quartzite
Quartzite is composed of an interlocking mass of quartz crystals with irregular boundaries,
produced by metamorphic processes. It may contain small crystals of mica or other iron bearing
mineral. This group includes aventurine quartz.
Cryptocrystalline Quartz
This is a group with microscopically small crystals of quartz embedded in opal, which is
itself hydrated quartz. The proportion of quartz to opal varies, and this variation is
reflected in differing densities, refractive indexes and other properties. The quartz crystals
are in a fibrous form, sometimes running approximately parallel to each other, sometimes
entwined.
All quartz in this group is a form of chalcedony or agate, although the term agate is often
used to describe the banded varieties of chalcedony. Many agates are naturally colored,
others are stained for decorative effect. Varieties include chrysoprase, cornelian (carnelian,
sard, sardonyx, onyx, plasma, bloodstone, myrickite. There are many differently named varieties
of agate depending on their appearance such as moss agate, fossilised wood. and other forms of
fossil. Jasper also belongs to this group, and contains impurities which cause heavy
pigmentation.
Opal
Opal is hydrated quartz which has formed a solidified gel, and itself has a number of
different varieties.
Gem Varieties of Quartz
The following are the main gem varieties of quartz. For details of each type, please take a
look at their individual pages, where available. Further information about the other varieties
will follow when time permits.
Agate
Amethyst
Aventurine
Bloodstone or Heliotrope
Citrine
Cairngorm
Cat's Eye
Chalcedony
Chrysoprase
Cornelian or Carnelian
Fire Opal
Hornstone
Jasper
Onyx
Opal
Petrified Wood
Plasma
Prase
Rock Crystal
Rose Quartz
Sard
Sardonyx
Siderite
Tiger's Eye
Technical Characteristics
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Chemical Composition:
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SiO2 - Silicon Dioxide
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Hardness:
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7
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Specific Gravity:
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2.65 - 2.66
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Refractive Index (R.I.):
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1.543-1.552 to 1.545-1.554
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Bi-refringence:
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0.009
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Optic Sign:
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Positive
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Optical Character:
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Uniaxial
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Crystal Structure:
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Trigonal
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