Uneven, Uneven, Gems, Sixth Edition (2006)
UnevenArthur Thomas, Gemstones (2009) Tough, UnevenArthur Thomas, Gemstones (2009) Tough, Brittle, Uneven
Al 2O 3Michael OâDonoghue , Gems, Sixth Edition (2006)
Mn 8Si 6O 15(OH,Cl) 10Ulrich Henn and Claudio C.