ConchoidalArthur Thomas, Gemstones (2009), ConchoidalArthur Thomas, Gemstones (2009)
Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Mg 3(Al,Cr) 2(SiO 4) 3Gemdat.org , Management Team (2012)
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.