Quartz is one of the most common minerals on earth and is well known in the gems world. Quartz is attractive and durable, as well as inexpensive. It can be cut and carved in many forms and sizes. Quartz is named after a Slavic word for "hard".
There are two main varieties of quartz, though they share the same chemical composition, silicon dioxide. Macrocrystalline quartz, includes stones like amethyst, aventurine, rock crystal, blue quartz, citrine, hawk's eye, prasiolite, quartz cat's eye, smoky quartz, rose quartz and tiger's eye. The quartz is mostly transparent to translucent. Cryptocrystalline quartz, with microscopically small crystals, is known as chalcedony, and includes agate, chrysoprase, bloodstone, jasper and carnelian. Crytocrystalline quartz is usually opaque or translucent.
The color of macrocrystalline quartz is as variable as the spectrum, but clear quartz is by far the most common color followed by white or cloudy. Purple (amethyst), pink (rose quartz), gray or brown to black (smoky quartz) are also common. Cryptocrystalline quartz varieties can be multicolored. Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous. Crystals are transparent to translucent; cryptocrystalline forms are usually translucent or opaque.
The colors of macrocrystalline quartz are as follows:
Rock crystal: Colorless. Material that can be cut is rare. Inclusions are of goethite, gold, pyrite, rutile or tourmaline. The luster is vitreous.
Smoky quartz: Brown to black, smoky gray. The coloring is caused by natural and artificial gamma rays. Frequent inclusions are rutile needles.
Amethyst: Purple, violet, pale red-violet. Amethyst is the most highly valued stone in the quartz group. The coloring agent is iron.
Amethyst Quartz: Violet with whitish stripes. Amethyst quartz is a more compact formation of amethyst, layered and striped with milky quartz.
Ametrine: Yellow and violet. Color-zoned quartz variety that consists half of amethyst and citrine.
Citrine: Light yellow to dark yellow, gold-brown. The coloring agent is iron. Many commercial citrines are heat-treated amethyst. Natural citrines are mostly pale yellow. If heat-treated they acquire a reddish tint.
Prasiolite: Leek-green. Prasiolite is not found in nature. Prasiolite is produced by heat treatment of amethyst or yellowish quartz.
Rose quartz: Strong pink, pale pink. Coloring agent is titanium. Traces of included rutile needles cause six-rayed stars when cut en cabochon. Larger clear stones can be faceted. Rose quartz crystals tend to be cloudy, which deepens the color. Transparent crystals are extremely rare.
Aventurine: Green, red-brown, gold-brown. Mostly dark green with metallic glittery appearance caused by included fuchsite (green mica), or red- to gold-brown caused by hematite leaves.