ConchoidalArthur Thomas, Gemstones (2009)
UnevenArthur Thomas, Gemstones (2009), UnevenArthur Thomas, Gemstones (2009), Uneven
Tourmaline is a series of several different minerals with unique chemical formulas.
FeS 2Michael OâDonoghue , Gems, Sixth Edition (2006) More from other references