Tourmaline :


The tourmaline mineral group is chemically one of the most complicated groups of silicate minerals.It is a complex silicate of aluminium and boron, but because of isomorphous replacement,its composition varies widely with sodium,calcium,iron,magnesium,lithium and other elements entering into the structure.

Tourmaline has a wide variety of colors.Usually,iron-rich tourmalines are black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are practically any color: blue, green, red, yellow, pink etc. Rarely, it is colourless. Bi-colored and multicoloured crystals are relatively common, reflecting variations of fluid chemistry during crystallisation. Crystals may be green at one end and pink at the other,or green on the outside and pink inside: this type is called watermelon tourmaline.Some forms of tourmaline are dichroic,in that they appear to change color when viewed from different directions.

Tourmaline is found in two main geological occurrences.Igneous rocks,in particular granite and granite pegmatite and in metamorphic rocks such as schist and marble. Schorl and lithium-rich tourmalines are usually found in granite and granite pegmatite. Magnesium-rich tourmalines, dravites, are generally restricted to schists and marble. Also, tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate.

Another unique variety that is also highly valued is chrome tourmaline,a rare type of dravite tourmaline from Tanzania which occurs in a very rich green color caused by chromium, the same element which causes the green in emerald. Of the standard Elbaite colors, generally blue indicolite gems are the most expensive, followed by green verdelite and pink to red rubellite. Ironically the rarest variety, colourless achroite, is not appreciated and is the least expensive of the transparent tourmalines.