ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
UnevenArthur Thomas, Gemstones (2009), UnevenArthur Thomas, Gemstones (2009)
SiO 2Ulrich Henn and Claudio C.
(Fe,Mn) 7Si 7O 21Ulrich Henn and Claudio C.