include amethyst (purple), citrine (yellow), milky quartz (cloudy, white
variety), rock crystal (clear variety), rose quartz (pink to reddish-pink
variety), and smokey quartz (brown to grey variety).
The word amethyst comes from the Greek amethustos meaning "not drunk".
Therefore, it has been considered a charm against intoxication. A legend
accounts for the origin of the stone. Supposedly, Bacchus, the god of wine
and conviviality, grew angry at a slight and swore revenge. He decreed that
the first mortal to come across his path was to be eaten by tigers.
Amethyst, a beautiful maiden on her way to worship at the shrine of Diana,
happened to be the victim. Diana, the huntress, changed Amethyst into
colorless quartz to protect her from the tigers. When Bacchus witnessed the
miracle, he repented and poured wine over the stone, staining it purple. The
wine failed to cover the entire stone evenly, and the feet and part of the
legs remained clear crystal. So, in keeping with the legend, amethyst
crystals are usually uneven in color with a colourless base.
Varieties - A variety of quartz, silicon dioxide, which appears to be dark
purple in transparent light. A transparent variety of quartz, silicon
dioxide, occurring in yellow to red-orange to orange-brown. The name is
derived from citron, which is French for lemon.
Treatments - Poor quality amethyst is often heat-treated to achieve a
desirable citrine colour.
History - The name "quartz" comes from the German "Quarz", which is of
Slavic origin (Czech miners called it krem). Other sources insist the name
is from the Saxon word "Querkluftertz", meaning cross-vein ore.
Quartz is the most common material identified as the mystical substance
maban in Australian Aboriginal mythology.
Roman naturalist Pliny the Elder believed quartz to be permanently frozen
ice. He supported this idea by saying that quartz is found near glaciers in
the Alps and that large quartz crystals were fashioned into spheres to cool
the hands. He also knew of the ability of quartz to split light into a
spectrum.
Nicolas Steno's study of quartz paved the way for modern crystallography. He
discovered that no matter how distorted a quartz crystal, the long prism
faces always made a perfect 60 degree angle.
Charles Sawyer invented the commercial quartz crystal manufacturing process
in Cleveland, OH. This initiated the transition from mined and cut quartz
for electrical appliances to manufactured quartz.
Quartz is one of the most common minerals in the Earth's continental crust.
It has a hexagonal crystal structure made of trigonal crystallized silica
(silicon dioxide, SiO2), with a hardness of 7 on the Mohs scale. Density is
2.65 g/cm³. The typical shape is a six-sided prism that ends in six-sided
pyramids, although these are often twinned, distorted, or so massive that
only part of the shape is apparent from a mined specimen. Additionally a bed
is a common form, particularly for varieties such as amethyst, where the
crystals grow up from a matrix and thus only one termination pyramid is
present. A quartz geode consists of a hollow rock (usually with an
approximately spherical shape) with a core lined with a bed of crystals.
Quartz is one of the world's most common crustal minerals and goes by a
bewildering array of different names. The most important distinction between
types of quartz is that of macrocrystalline (individual crystals visible to
the unaided eye) and the microcrystalline or cryptocrystalline varieties
(aggregates of crystals visible only under high magnification). Chalcedony
is a generic term for cryptocrystalline quartz. The cryptocrystalline
varieties are either translucent or mostly opaque, while the transparent
varieties tend to be macrocrystalline.
Although many of the varietal names historically arose from the color of the
mineral, current scientific naming schemes refer primarily to the
microstructure of the mineral. Colour is a secondary identifier for the
cryptocrystalline minerals, although it is a primary identifier for the
macrocrystalline varieties. This does not always hold true.
Not all varieties of quartz are naturally occurring. Prasiolite, an olive
colored material, is produced by heat treatment; natural prasiolite has also
been obeserved in Lower Silesia in Poland. Although citrine occurs
naturally, the majority is the result of heat-treated amethyst. Carnelian is
widely heat-treated to deepen its color.
Because natural quartz is so often twinned, much quartz used in industry is
synthesized. Large, flawless and untwinned crystals are produced in an
autoclave via the hydrothermal process: emeralds are also synthesized in
this fashion.
Quartz occurs in hydrothermal veins and pegmatites. Well-formed crystals may
reach several metres in length and weigh hundreds of kilograms. These veins
may bear precious metals such as gold or silver, and form the quartz ores
sought in mining. Erosion of pegmatites may reveal expansive pockets of
crystals, known as "cathedrals."
Quartz is a common constituent of granite, sandstone, limestone, and many
other igneous, sedimentary, and metamorphic rocks.
Tridymite and cristobalite are high temperature polymorphs of SiO2 which
occur in high silica volcanic rocks. Lechatelierite is an amorphous silica
glass SiO2 which is formed by lightning strikes in quartz sand.
Quartz is also a type of piezoelectric crystal that creates electricity
through a process called piezoelectricity when mechanical stress is put upon
it. One of the earliest uses for a quartz crystal was a phonograph pickup.
Today, one of the most ubiquitous piezoelectric uses of quartz is as a
crystal oscillator -- in fact these oscillators are often simply called
"quartzes". The same principle is also used for very accurate measurements
of very small mass changes by means of the quartz crystal microbalance.
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