Gemstones (2009), UnevenArthur Thomas, Gemstones (2009), Brittle, Uneven
ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
ThCa 2Si 8O 20Walter Schumann , Gemstones of the world (2001) More from other references
SiO 2Ulrich Henn and Claudio C.