Gems, Sixth Edition (2006)
ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
SiO2 (with a high level of nickel impurities )
SiO 2(+Al 2O 3)Walter Schumann , Gemstones of the world (2001)