Mineral Tourmaline containing boron silicate in crystalline form comes with associated elements like Aluminum, Sodium, Iron, Magnesium, Lithium and Potassium. Tourmaline is listed as a semi-precious stone. This gemstone comes in wide ranging colors. Tourmaline does not occur as a lone mineral, but as a group of many closely connected minerals, of which the three most familiar are, Dravite, Elbaite, and Schorl.
(Ca, K, Na, )
Derivation of Name
It derives its name from the Sinhalese word "Turmali" or "Thoramalli", which is used for various gemstones occurring in Sri Lanka.
Tourmaline occurs in granite and granite pegmatites and also in metamorphic rocks like marble and schist. Lithium-rich tourmalines and schorl are often located in granite and granite pegmatite. Tourmalines rich in magnesium are often limited to marble and schists.
Three more members of the group that are at times used as gemstones include Dravite, Liddicoatite and Schorland. Dravite has brown color and is hardly ever clear enough for being utilized as a gem. They may be subjected to heat treatment to reduce darkness of color and are occasionally cut into gems.
Tourmaline of gem quality gets mined mainly in Brazil. Some material appropriate for use as gem is available in Sri Lanka. Other than Brazil, mines of tourmaline are also found in Tanzania, Kenya, Nigeria, Madagascar, Mozambique, Afghanistan, Pakistan, Namibia, Malawi and Sri Lanka.
Tourmaline appears as a six member ring cyclosilicate with a trigonal crystal structure. It comes as long, slim to thick, columnar and prismatic crystals which generally have a triangular cross-section, frequently with curved striated faces. The way crystals terminate at their ends is now and then asymmetrical and called hemimorphism. Tiny slim prismatic crystals are generally found as fine-grained granite and called Aplite, habitually forming radial daisy-like patterns. Tourmaline is notable for its three-sided prisms. No other widespread mineral comes with three sides
By and large, color of iron-rich tourmalines are bluish-black to black to deep brown, whilst varieties rich in magnesium come in brown to yellow color, but lithium-rich tourmalines come in about any color: blue, green, yellow, pink, red etc. Rarely, it comes colorless.
Mohs scale hardness