Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
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Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.
NaFe 3+ 3 Al 6 (BO 3 ) 3 Si 6 O 18 (O,F,OH) 4 Anthony et al, Handbook of mineralogy (2001)