Tourmaline – natural and collector minerals

Tourmaline – a diverse group of borosilicate minerals

Tourmaline is not a single mineral, but a large group of chemically related borosilicates with highly variable compositions. Individual tourmaline species may contain iron, magnesium, sodium, lithium, calcium, manganese and other elements. This complex chemistry is responsible for the exceptionally broad range of colors, including black, brown, green, blue, pink, red, yellow and multicolored crystals.

Tourmalines typically form elongated prismatic crystals with pronounced longitudinal striations. Their cross-sections are often triangular to rounded triangular. Some crystals display strong color zoning, with shades changing from the center toward the outer edge or along the length of the crystal. A well-known example is watermelon tourmaline, which has a pink center surrounded by a green outer zone.

The best-known members of the tourmaline group include black schorl, brown to yellow-brown dravite, colorful elbaite and strongly zoned liddicoatite. Transparent colored tourmalines are used as gemstones, while well-developed crystals, druses and crystals embedded in their host rock are valued as mineralogical specimens.

How tourmaline forms

Tourmalines form in several geological environments, but they are especially common in boron-rich granitic pegmatites. Boron, which is essential for tourmaline formation, becomes concentrated in residual melts and hydrothermal fluids during the final stages of magma crystallization. Under these conditions, large and well-developed crystals may grow.

Tourmaline also occurs in metamorphic rocks such as gneiss, mica schist and marble. Black schorl is a common component of granites and pegmatites, whereas dravite forms more frequently in magnesium-rich rocks. Lithium-bearing elbaites are especially characteristic of highly evolved pegmatites.

Changes in chemical composition during crystal growth produce color zones. A single crystal may therefore contain several colors or gradual transitions between them. Tourmaline is also pyroelectric and piezoelectric, meaning that electrical charges may develop at its ends when the crystal is heated, cooled or subjected to mechanical pressure.

Major tourmaline localities

Tourmaline occurs at numerous localities around the world. Individual deposits differ in chemical composition, color, crystal size and overall specimen quality.

• Brazil: particularly the state of Minas Gerais, famous for green, pink, blue and multicolored elbaites.
• Afghanistan and Pakistan: important sources of transparent pink, green, blue and bicolored crystals.
• Madagascar: renowned for colorful elbaites, liddicoatites and strongly zoned crystals.
• Mozambique and Namibia: important sources of high-quality gem and collector tourmalines in various colors.
• United States: historic deposits in Maine and California, known for pink, green and watermelon tourmalines.