ConchoidalWalter Schumann, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Brittle, Metallic
Uneven, Uneven, Gems, Sixth Edition (2006)
Be 2BO 3(OH)Ulrich Henn and Claudio C.
Al 2O 3Michael OâDonoghue , Gems, Sixth Edition (2006)