Not required for EPS2 students
Distinguished from topaz and andalusite by stronger D. R. Tourmalines are widespread and abundant complex boron and aluminum silicate minerals. they form slender, three, six, or nine sided prismatic crystals in parallel or radiating groups. luster is vitreous to resinous, streak is uncolored, The alkali tourmalines , which contain sodium , potassium, or lithium, are pink (rubellite), green (so called 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 crystals are most commonly found in 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. Most gem material comes from Ceylon gem gravels and from pegmatites found in Madagascar , the Ural Mountains, and Maine. Tourmaline is the birthstone for October.