Uneven, Uneven, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Conchoidal, Brittle
Subconchoidal, ConchoidalArthur Thomas, Gemstones (2009), Conchoidal, Brittle
Fe 2O 3Arthur Thomas , Gemstones (2009)
Na 3(Na,K)Al 4Si 4O 16Ulrich Henn and Claudio C.