ConchoidalWalter Schumann, Gemstones of the world (2001), ConchoidalWalter Schumann, Gemstones of the world (2001)
Uneven, Uneven, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Conchoidal, Brittle
SiO 2Walter Schumann , Gemstones of the world (2001)
Fe 2O 3Arthur Thomas , Gemstones (2009)