Properties of selected silica minerals

name colour lustre Mohs hardness specific gravity

coesite colourless vitreous near 8 2.9–3.0

cristobalite (low-temperature form) white or milky vitreous 6½ 2.2–2.3

lussatite (fibrous low-cristobalite) white, gray, bluish, yellowish 2.0–2.1

opal (submicrocrystalline low-cristobalite) white to colourless; milky to bluish white; variable pale shades vitreous to subvitreous, resinous, or pearly 5½–6½ 2.0–2.3

quartz (low-temperature form) variable vitreous to greasy (coarse-grained); waxy to dull (fine-grained) 7 (a hardness standard) 2.65

tridymite (low-temperature form) colourless to white vitreous 7 2.26

name habit or form fracture or cleavage refractive indices

coesite transparent, fine-grained matrix material alpha = 1.593

gamma = 1.597

cristobalite (low-temperature form) small octahedral crystals; also, see below no apparent cleavage omega = 1.484

epsilon = 1.487

lussatite (fibrous low-cristobalite) translucent to opaque, fibrous crusts and botryoidal aggregates

opal (submicrocrystalline low-cristobalite) submicrocrystalline aggregates; globular or kidney-like crusts; irregular concretions conchoidal fracture n = 1.435–1.455

quartz (low-temperature form) prismatic and rhombohedral crystals; massive conchoidal fracture omega = 1.544

epsilon = 1.553

tridymite (low-temperature form) thin, transparent plates (pseudomorphs of high-tridymite) conchoidal fracture alpha = 1.468–1.479

beta = 1.469–1.480

gamma = 1.473–1.483