Conchoidal, ConchoidalWalter Schumann, Gemstones of the world (2001), Brittle
Subconchoidal, ConchoidalArthur Thomas, Gemstones (2009), Conchoidal, Brittle
Be 2SiO 4Michael OâDonoghue , Gems, Sixth Edition (2006) More from other references
Na 3(Na,K)Al 4Si 4O 16Ulrich Henn and Claudio C.