ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
UnevenArthur Thomas, Gemstones (2009) Tough, UnevenArthur Thomas, Gemstones (2009) Tough, Brittle, Uneven
SiO 2Ulrich Henn and Claudio C.
Mn 8Si 6O 15(OH,Cl) 10Ulrich Henn and Claudio C.