Uneven, Uneven, Gems, Sixth Edition (2006)
Uneven, Uneven, ConchoidalWalter Schumann, Gemstones of the world (2001) More from other references, Conchoidal, Brittle
Al 2O 3Michael OâDonoghue , Gems, Sixth Edition (2006)
Fe 2O 3Arthur Thomas , Gemstones (2009)