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