Conchoidal, Brittle, Conchoidal
ConchoidalArthur Thomas, Gemstones (2009), ConchoidalArthur Thomas, Gemstones (2009), Brittle, Conchoidal
AlO(OH)Walter Schumann , Gemstones of the world (2001) More from other references
ZrO 2Arthur Thomas , Gemstones (2009)