Chemical composition -- The tourmaline family is a group of closely related minerals with complex chemical formulas containing aluminum borosilicates and varying quantities of sodium, magnesium, calcium, iron, chromium, manganese, potassium, etc. They vary widely in color and physical properties but share a common crystal structure. Found in many locations throughout the world, they produce striking, relatively durable gems and show the widest color variations of any gemstone.
Color -- Extremely wide color range.
Optics -- R.I. 1.63-1.64, varying with the composition. Prominent pleochroism and pencil-shaped crystal often dictate cutting green tourmaline in long step cuts with steep pavilion ends to minimize black C axis effect.
Durability -- Hardness 7.0-7.5.
Crystal structure -- Hexagonal.
Specific Gravity -- 3.0-3.25, varying with the composition.
Sources -- Very widespread. Notable deposits include California, Maine, Brazil, Namibia, Madagascar, Mozambique, Zambia, Afghanistan.