Hardness : 7.5
Specific Gravity : 3.06
Refractive index : 1.62 - 1.6
Doubly Refracting (bi-refraction) : 0.018
Tourmaline, complex borosilicate mineral with varying amounts of aluminum, iron, magnesium, sodium, lithium, potassium, and sometimes other elements, used as a gem. It occurs in prismatic crystals, commonly three-sided, six-sided, or nine-sided, and striated vertically.
Different crystal forms are usually present at opposite ends of the vertical axis. The luster is vitreous. Colors are red and pink (Rubellite), blue (Indicolite or Brazilian sapphire), green (Brazilian emerald), yellow, violet-red, and black (Schorl). Colorless varieties are called Achroite.
Two or more colors may occur in the same stone, the colors being arranged in zones or bands with sharp boundaries between them. Some Brazilian stones have a red core with a green exterior, separated by a colorless band; some stones from California are green within and red outside. The variations in color are, of course, dependent on the variations in chemical composition. Tourmalines are found in pegmatite veins in granites, gneisses, schists, and crystalline limestone. Sources of the gem include Elba, Madagascar, Myanmar, Sri Lanka, the Urals, Siberia, Brazil, and Maine, Connecticut, and California in the United States.