ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
ConchoidalWalter Schumann, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Brittle, Metallic
SiO 2(+Al 2O 3)Walter Schumann , Gemstones of the world (2001)
Be 2BO 3(OH)Ulrich Henn and Claudio C.