a very hard, piezoelectric, rhombohedral mineral, (Na,Ca)(Mg,Fe,Al,Li)Al(BO)(SiO)(OH), of various colors, used as a gem and in making pressure gauges
See tourmaline in American Heritage Dictionary 4
(to͝orˈmə-lĭn, -lēnˌ)
noun
A complex crystalline silicate containing aluminum, boron, and other elements, used in electronic instrumentation and, especially in its green, clear, and blue varieties, as a gemstone.