Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Splintery, Granular, Splintery, Granular, Gemstones (2009)
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.
Ca 2(Mg,Fe) 5(OH) 2Walter Schumann , Gemstones of the world (2001) More from other references