ConchoidalWalter Schumann, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Brittle, Metallic
Gemstones (2009), UnevenArthur Thomas, Gemstones (2009)
Be 2BO 3(OH)Ulrich Henn and Claudio C.
Al 2SiO 5Arthur Thomas , Gemstones (2009)