M759 - These unusual tourmaline specimens from Mexico have an interesting fine grained crystal structure.

Black Tourmaline crystal. Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminum, iron, magnesium, sodium, lithium, or potassium. Classified as a semi-precious stone. Also called schorl.

All tourmalines can be divided in three species: Dravite: dark yellow to brownish balck-dravite. Schorl: Bluish or brownish black to Black-schorl. Elbaite: Red, pinkish-red-rubellite variety, dark black-schorl, light blue to bluish green, colorless-archroite variety.

The most common species of tourmaline is schorl. It may account for 95% or more of all tourmaline in nature.

Tourmaline belongs to the trigonal crystal system and occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section.

Tourmaline is found in granite and granite pegmatites and in metamorphic rocks such as schist and marble. Tourmaline is a durable mineral and can be found in minor amounts as grains in sandstone and conglomerate. Some tourmaline gems, especially pink to red colored stones, are altered by irradiation to improve their color. Irradiation is almost impossible to detect in tourmalines, and does not impact the value. Heavily-included tourmalines, such as rubellite and Brazilian paraiba, are sometimes clarity-enhanced.

Black tourmaline has a hardness of 7 to 7-1/2 on the Mohs hardness scale.