Uneven, Uneven, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Conchoidal, Brittle
Conchoidal, ConchoidalArthur Thomas, Gemstones (2009), Brittle
Fe 2O 3Arthur Thomas , Gemstones (2009)
Al 6B 5O 15(F,OH) 3Ulrich Henn and Claudio C.