The mineral, Tourmaline, is widespread and abundant complex boron and aluminum SILICATE MINERAL. Tourmalines form slender, three-, six-, or nine-sided prismatic crystals (hexagonal system) in parallel or radiating groups. Hardness is 7 to 7.5, luster is vitreous to resinous, streak is uncolored, and specific gravity is 3.0 to 3.2. The alkali tourmalines, which contain sodium, potassium, or lithium, are pink (rubellite), green (Brazilian emerald), or colorless (achroite), whereas magnesium tourmaline is yellow brown to brownish black (dravite), and iron tourmaline is deep black (schorl). Color gradation along the lengths of crystals is common, with pink usually found at one end grading into green at the other. Tourmalines develop an electrical charge when heated or deformed, and slabs cut perpendicular to the long axis can polarize light. The best-developed tourmaline crystals are found in granite pegmatites. Crystals are also found in limestones altered by granitic intrusions and, because of high resistance to weathering, in detrital deposits and sedimentary rocks. Transparent colored stones and opaque black crystals are faceted as gemstones. Tourmalines are found on the island of Elba, in Sri Lanka, Madagascar, and the Ural Mountains, and in the state of Maine in the United States.