Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Conchoidal, Conchoidal, Gemstones of the world (2001)
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.
SiO 2 · nH 2OWalter Schumann , Gemstones of the world (2001) More from other references