ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Splintery, UnevenArthur Thomas, Difficult, Fibrous
SiO 2Walter Schumann , Gemstones of the world (2001)
NaAlSi 2O 6Ulrich Henn and Claudio C.