ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
UnevenArthur Thomas, Gemstones (2009), UnevenArthur Thomas, Gemstones (2009)
Al 2Be 3Si 6O 18Martin H.
(Fe,Mn) 7Si 7O 21Ulrich Henn and Claudio C.