Amethyst, purple color stone is a very popular gemstone. It has been associated with royalty since ages. But what it is in geological terms? It is a variety of quartz which is chemically known as silica oxide (SiO2), a compound of silica (46%) and oxygen (54%). Silicon and oxygen are the most common constituents of the crust of the earth. Quartz, a tectosilicate, is the second most commonly found mineral in the crust following feldspar. Weathering resistant mineral is found in all three types of rocks in different forms. In ancient world, quartz crystals were considered as frozen ice. The use of rock crystal quartz as ornament dates back several centuries. Prior to that weapons and tools were carved out of these crystals.

Properties

Generally quartz is white / colorless. However, there are some colored quartz varieties. Cleavage is absent from the mineral with vitreous luster. Some times quartz with resinous luster is also found. Quartz with 7 point hardness (Mohs' scale) is found in metamorphic and igneous rocks as anhedral (shapeless) grains. Rounded grains of quartz occur in sedimentary clastic rocks. Specific gravity of quartz is 2.65. Streak of pure quartz is white and in colored varieties, streak is of the color of mineral. Fractures are well developed conchoidal. Quartz belongs to trigonal crystal system. Quartz available in prismatic crystals can scratch glass. Simple crystals are either left handed or right handed. Quartz is brittle to tough. Thermo-conductivity of mineral is high. It is insoluble in water.

Varieties

Two forms of quartz are low/alpha quartz that is stable up to 573 o C and high/beta quartz which is stable above 573 o C. Alpha is trigonal and beta is hexagonal in structure.

Quartz can be crystalline or cryptocrystalline. Amethyst, citrine, rose quartz and smoky quartz are crystalline varieties. They have large crystals. Cryptocrystalline varieties have small crystals that can be viewed under microscope. These varieties are usually waxy to dull. They are translucent / opaque. Agate, jasper, and onyx are example of crypto crystalline quartz.

Purest form of quartz is rock crystal. This transparent variety of quartz is used to manufacture ornaments. Deep-sea diving vessels also use it for windows.

Semi precious amethyst is bluish violet/purple and transparent. The gemstone gets its color from manganese.

Pale yellow or pale brown quartz is called citrine and smoky quartz.  Smoky quartz is usually transparent. Smoky quartz contains carbon dioxide in gaseous and liquid form. Color of smoky quartz varies from black to smoky brown. Crystals of smoky quartz also have inclusions of rutile. Citrine is a gemstone of yellow to orange color.

Quartz with Ti, Na and Li impurities is pale pink and known as rose quartz. Titanium gives pink / rose red color to the quartz. Translucent rose quartz contains very coarse grains. Rose quartz, a constituent mineral of pegmatities, is found in Brazil, USA, India and Madagascar.

Black variety of the mineral is morion.

White quartz with a number of microscopic air cavities is very common and called milky quartz. Luster of milky quartz is greasy.

The mineral with fibrous structure is called cat's eye. Its cut faces show opalescent luster.

Quartz featuring spangles of hematite, mica and so on is aventurine quartz.

Ferruginous quartz, brownish / reddish quartz, has iron oxides.

Star-quartz contains colored / whitish radiations.

Rare variety of quartz in Berlin blue / indigo color is called sapphire quartz.

Agate is either banded or has irregular clouds. Varieties of agate include blood stone (green showing red speckles) carnelian (yellow to orange) chrysoprase (green), sard (yellow to brown), and sardonyx (sard with bands),

Onyx depicts alternate layers of red and black, white and black / white and red colors. This variety is used to carve cameos. The layers are flat. Some types of onyx are used for ornaments, lamps, bookends, and vases. Major producers of onyx are South America and India.

Jasper is opaque quartz in blue, dark green, red and yellow colors. It is impure and massive form of quartz.

Chalcedony, cryptocrystalline quartz, is translucent/transparent. Its luster is waxy. The mineral is available in different colors including blue, black, white, dark brown and pale yellow. It occurs in cavities in rocks.

Other varieties are Arkansas candle, blue quartz, capped quartz, cotterite, bull quartz, chert, oil quartz, and hedgehog stone

Occurrence

Quartz, a common rock forming mineral, occurs in cavities, veins and joints. Quartz is main component of acid igneous plutonic rocks viz. granites. Quartz as phenocrysts occurs in hypabyssal rocks viz. rhyolites. Dominant component of sands is also quartz.

Quartzite is a rock mainly (90%) consisting of quartz. It is nonporous and very hard. Grains in the rock can not be distinguished because they are tightly arranged. A rock of quartz pebble and sand is called quartz-conglomerate. Gneiss, a common metamorphic rock is composed of quartz and feldspar.

Uses

Colored varieties are used in jewelry. Gemstone varieties include rose quartz, smoky quartz, citrine and amethyst.

Its piezoelectric properties make it useful for depth-sounding apparatus, wave stabilizers, resonators, oscillators and pressure gauges. Piezoelectric means development of negative and positive charges on prism edges if crystal is under tension / pressure.

Quartz is also used in communication devices (radio, TV, etc.) and crystal-controlled watches and clocks. Quartz watches are common and they run on a miniature battery that can be replaced.

It is also used in spectrographic and prism lenses, and heat ray lamps as it is transparent in UV rays and can rotate the plane of polarization of light.

To manufacture precision instruments, refractory, abrasives, paints and glass, quartz is used. Quartz sand / silica sand are also used for foundry molds and ceramics. Quartz is used in many other processes and products like scouring soaps, paints, and smelting operations (as acid flux).

Sandstone, a building stone, is principally contains quartz. Besides, sandstone is used for grindstones, millstones, and whet stones.

Sensitive weighing devices use quartz fiber.

Quartz and Health