ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Uneven, Uneven, Gems, Sixth Edition (2006)
SiO 2 · nH 2OWalter Schumann , Gemstones of the world (2001)
Al 2O 3Michael OâDonoghue , Gems, Sixth Edition (2006)