Quartz
Composition: SiO₂. Crystal system: hexagonal. Hardness: 7. Specific gravity: 2.65 to 2.66 in crystals. Luster: vitreous, also waxy, greasy, and dull. Color: most often colorless, brown, yellow, violet; sometimes green, red, blue, and black; cryptocrystalline varieties often variously colored by impurities. Streak: white. Cleavage: indistinct. Fracture: conchoidal to splintery. Tenacity: brittle to tough. Diaphaneity: transparent to opaque. Refractive index: 1.544 to 1.553.
The quartz family gemstones can be divided into two groups for purposes of description. The first group is the crystalline varieties, or those quartz varieties that commonly occur in distinct crystals. The second group is the cryptocrystalline varieties, or those quartz varieties that occur as irregular masses that are composed of many microscopic crystals. The crystalline varieties are usually much more transparent and are most often seen as faceted stones. The cryptocrystalline varieties vary from subtransparent to opaque and are almost always cut as cabochons.