_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.