Uneven, Uneven, Gems, Difficult, Fibrous
ConchoidalAnthony et al, Handbook of mineralogy (2001), ConchoidalAnthony et al, Handbook of mineralogy (2001), Brittle, Conchoidal
Al 2O 3Arthur Thomas , Gemstones (2009)
CaBe 2(PO 4) 2Michael OâDonoghue , Gems, Sixth Edition (2006)