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