South Dakota State Mineral
Quartz is the most common mineral found on the Earth’s surface, and the second most abundant mineral in the Earth’s crust after feldspar. Quartz makes up an estimated 12 percent of the continental crust; it makes up less of the oceanic crust. Made of silicon and oxygen, quartz occurs in basically all mineral environments and is a component of many rocks and the inner layer of many geodes.
Rock crystal, the most common gem variety of quartz crystal, is pure quartz, and like all quartz, is formed from the two most abundant elements in the Earth’s crust, silicon and oxygen.
Quartz is found in an impressive range of varieties and colors, including purple, rose, black, yellow, brown, green, and orange, and can also be colorless or multicolored. The main varieties of this stone are chalcedony, agate, amethyst, citrine, rose quartz, rock crystal, smoky quartz, milky quartz, rutilated quartz, onyx, tiger’s eye, and aventurine. Rose quartz is the translucent pink variety, and research indicates that rose quartz’s color may be due to microscopic inclusions of a fibrous mineral, such as the mineral Dumortierite or a related species. Quartz varieties are classified as macrocrystalline, meaning their crystals are large and visible to the naked eye; microcrystalline, meaning their crystals can be seen with a microscope; or cryptocrystalline, meaning crystals are so small they are difficult to make our even with a microscope. Microcrystalline and cryptocrystalline quartz are termed chalcedony.
Rose quartz was designated the state mineral of South Dakota in 1966. Discovered near Custer in the late 1880s, it is still mined in the southern Black Hills. This mineral is used primarily for ornaments and jewelry.
Forms of quartz are an official symbol in two other states. Quartz is the official state crystal of Arkansas, while smoky quartz is the official state gem of New Hampshire.
The word "quartz" comes from the German quarz, which is of Slavic origin (Czech miners called it křemen) and means "hard."
FORMATION AND OCCURRENCE
Quartz commonly occurs with igneous rocks such as granite, sedimentary rocks such as sandstone and shale, and metamorphic rocks like schist, gneiss, and quartzite. Its resistance to weathering also makes it common in stream sediments and residual soils. Quartz occurs in hydrothermal veins along with ore minerals. Large crystals of quartz are found in pegmatites.
Giant quartz crystals, along with large specimens of dolomite, talc, and other minerals, occur at Farm Natas/Verloren in the Hakos Mountains, Central Namibia. What is thought to be the largest quartz specimen on display was mined on the Otjua farm in Namibia; it can be visited in the Kristallgalerie in Swakopmund, Namibia.
Quartz is extraordinarily common and is found all over the world. The following is a list of some of the more important localities.
Early civilizations believed that quartz crystals were permanently frozen ice. Quartz’s high thermo-conductivity, which makes it feel cool to the touch, may have added to this belief. Regardless of what early people may have believed, historical records show the use of rock crystal for decoration and jewelry for at least 4,000 years. Tools and weapons were made from rock crystal long before it was used for decoration and jewelry.
For centuries in Europe and the Middle East, the various varieties of quartz were the most common semi-precious stones used in jewelry and carving. Valuable objects including engraved gems, cameos, and extravagant vases and vessels were carved from the stone.
Nicolas Steno, who is considered the father of geology, paved the way for modern crystallography with his studies of quartz in the 17th century. Steno discovered that no matter how distorted a quartz crystal, the long prism faces always made a perfect 60-degree angle.
Sand is composed of tiny quartz pebbles and is the primary ingredient in manufactured glass. Transparent rock crystal is used in the study of optics. Quartz is also used as an abrasive for sandblasting, grinding glass, and cutting soft stones. Quartz is essential in the computer industry, since silicon semiconductors are made from the mineral.
Quartz crystals are piezoelectric, which means they develop an electric charge with mechanical stress. This property was employed in phonograph pickups using quartz crystals, and today quartz is used as a crystal oscillator in quartz clocks, watches, radios, and pressure gauges. In addition, quartz is important in the production of soaps and ceramics.
In the gem industry, many varieties of quartz are cut as faceted stones. One of the most popular gems, quartz is often cut as a brilliant round to maximize the color. Amethyst is the most popular quartz gem, and citrine is the most valuable. Rose quartz, smoky quartz, rock crystal, and aventurine are also cut into gems. The black and white combination of rock crystal and onyx was popular in art deco jewelry design. Some believe that a clear quartz crystal pendant will bring the wearer good luck. Colorless stones can be heat-treated, irradiated, or dyed to enhance the color.
Quartz specimens are very popular with mineral collectors. Some collectors specialize their entire collections on different types of quartz.
Quartz has long been thought to have mystical and magical properties. Rock crystal and smoky quartz were once used for crystal balls by fortunetellers and witches. But since large, flawless specimens of rock crystal are rare, crystal balls are now made of glass. During the late 1980s and early 1990s, metaphysical uses and applications of rock crystal resulted in an increase in its production and processing. The metaphysical market used raw crystal in jewelry, personal power and healing devices, and as charms. Additionally, spheres, skulls, pyramids, and other metaphysical objects were made from rock crystal. The metaphysical market has declined from its peak and now appears to have stabilized.
One of the first gems to be synthetically grown on a large scale was quartz. This industry was primarily developed during World War II to supply crystals for radios. Today synthetic quartz is used extensively in the electronics industry.
Chemical Formula: SiO2
Crystal Structure: Trigonal
Hardness (Mohs): 7; lower in impure varieties
Color: Clear (in pure form)
Birefringence: +0.009 (B-G interval)
Refractive Index: 1.544-1.553 - Dr +0.009 (B-G interval)
Density: 2.65 constant; variable in impure varieties
Some data courtesy of the Mineralogical Society of America
Author: World Trade Press