Lepidolite

Chemical Formula : K(Li,Al)₃(Si,Al)₄O₁₀(F,OH)₂
Hardness : 2.5 on cleavage, 4 across cleavage
Cleavage : One, perfect
Luster : Vitreous to pearly
Color : Pale lilac, rarely colorless, pale yellow or pale gray.
Streak : Whtie to gray
Density : 2.8-2.9
Diagnostic Features and Trivia : This member of the mica group
is easily identified by its stereotypical color and cleavage when found in large crystals. Otherwise, lepidolite resembles a mass of small, purple, mica flakes.
Crystal System and Habit : Monoclinic; uncommon crystals are pseudo-hexagonal prisms, but commonly found in medium to fine grained crystalline aggregates.
Twinning : None
Polymorphs : None
Index of Refraction : alpha=1.525-1.548; beta=1.551-1.585; gamma=1.554-1.587