Conchoidal, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Conchoidal, Brittle
Gems, Sixth Edition (2006)
CuSiO 2(OH) 2Michael OâDonoghue , Gems, Sixth Edition (2006) More from other references
SiO2 (with a high level of nickel impurities )