ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Handbook of mineralogy (2001), Difficult, Fibrous
SiO 2Walter Schumann , Gemstones of the world (2001)
Ca 6Si 6O 17(OH) 2Michael OâDonoghue , Gems, Sixth Edition (2006)