Uneven, Sub-Conchoidal, UnevenWalter Schumann, Gemstones of the world (2001) More from other references, Brittle, Metallic
Gems, Sixth Edition (2006)
ZnCO 3Michael OâDonoghue , Gems, Sixth Edition (2006) More from other references
SiO2 (with a high level of nickel impurities )