tourmalin

Tourmaline is a large, complex family of borosilicate minerals belonging to the cyclosilicate group. It is notable for its wide range of colors and its occurrence in a variety of geological environments. The name is often spelled tourmalin in older texts; the mineral is frequently treated as a single group with many distinct varieties.

Chemistry and structure: Tourmaline has a general XY3Z6O18(BO3)3(OH)3 structure, with three cation sites (X, Y, Z) that can host a variety of elements such as sodium, calcium, lithium, iron, magnesium, aluminum, and more. Trace elements in the crystal lattice give each stone its color. Crystals are typically elongated prismatic and show strong pleochroism, reflecting their trigonal (often described as hexagonal) crystal system and vertical striations.

Color varieties and names: The mineral group includes rubellite (red to pink), verdelite (green), indicolite (blue), elbaite (often multicolored), and paraiba tourmaline (blue-green with copper). Watermelon tourmaline refers to stones with pink centers and green exteriors. Many tourmalines are gem-quality and used in jewelry.

Occurrence and formation: Tourmaline forms in igneous pegmatites and in metamorphic rocks, commonly associated with quartz, feldspar, and micas. Major producing regions include Brazil, Africa (notably Namibia, Kenya, Madagascar), Afghanistan, Pakistan, and Sri Lanka, among others.

Properties and uses: It has a hardness of about 7–7.5 on the Mohs scale and a vitreous luster, ranging from transparent to opaque. Tourmaline is both pyroelectric and piezoelectric and displays pleochroism. It is widely mined for use as a gemstone, with heat treatment employed in some cases to modify color. The name is thought to derive from the Sinhalese term turamali, meaning “mixed color stones.”