Gemstones of the world (2001), Difficult, Splintery
Uneven, Conchoidal, ConchoidalArthur Thomas, Gemstones (2009)
Nephrite: Ca2(Mg,Fe)5Si8O22(OH)2 Jadeite: NaAlSi2O6
Tourmaline is a series of several different minerals with unique chemical formulas. See The chemical formula of Tourmaline for details.