Uneven, Uneven, Gems, Difficult, Fibrous
ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Al 2O 3Arthur Thomas , Gemstones (2009)
SiO 2(+Al 2O 3)Walter Schumann , Gemstones of the world (2001)