Quartz (hardness: 7 Mohs)
Silicon dioxide or silica
Specific gravity: 2.65
Coarsely crystalline varieties of silica—
_Rock crystal_: Colorless
_Amethyst_: Purple
_Citrine_: Yellow to amber
_Morion_: Black
_Smoky quartz or cairngorm_: smoky gray to brown
_Rose quartz_: Translucent pink
_Green quartz or praziolite_: Green
Cryptocrystalline varieties of silica—
_Chalcedony and Jasper_ (_variable_)
_Agate_: Bull’s eye agate, Iris or fire agate, Onyx, Sardonyx.
_Bloodstone or heliotrope._ _Carnelian._ _Chrysoprase._ _Moss agate._
_Plasma._ _Prase._ _Sard._ _Jasper._
Spinel (hardness: 8 Mohs)
Magnesium aluminum oxide
Specific gravity: 3.58-4.06
_Balas ruby_: Red
_Almandine spinel_: Purple red
_Rubicelle_: Orange
_Sapphire spinel and ghanospinel_: Blue
_Chlorspinel_: Green
[Illustration: Candelabra: white quartz, blue-capped red elbaite, and
tan albite, California.]
Topaz (hardness: 8 Mohs)
Aluminum silicate fluoride hydroxide
Specific gravity: 3.5-3.6