Tourmaline

Tourmaline is not a single mineral but a group of related boron silicates that share a complex chemistry and a distinctive trigonal crystal structure. It is celebrated for spanning the entire color spectrum, and a single crystal can display multiple colors, as in watermelon tourmaline with a pink core and green rind.

The most common species is black schorl, while the colorful gem varieties are mostly elbaite. Named color varieties include rubellite (red/pink), indicolite (blue), verdelite (green) and the electric copper-bearing Paraiba.

Tourmaline crystals are typically long and striated with a rounded-triangular cross section, a strong identification clue. They are also pyroelectric and piezoelectric, generating electric charge with heat or pressure.

Tourmaline forms most abundantly in granitic pegmatites, where boron-rich residual fluids crystallize large, well-formed crystals along with minerals like lepidolite, quartz and feldspar. Gem-quality elbaite comes mainly from these pegmatites.

It also occurs in metamorphic rocks such as schist and marble, and as a durable accessory mineral in granites. Because it is hard and resistant, tourmaline survives weathering and is found in alluvial gem gravels.

Major sources include Brazil, Afghanistan, Pakistan, Madagascar, Nigeria, Mozambique and the United States (Maine and California). The vivid Paraiba type, colored by copper and manganese, was first found in Brazil and later in Africa.

Look for long prismatic crystals with strong lengthwise striations and a rounded triangular cross section, a near-diagnostic habit. Tourmaline is hardness 7-7.5 with a vitreous luster and shows strong dichroism, often appearing darker down the length of the crystal.

Color zoning along or across the crystal (as in watermelon tourmaline) is highly characteristic. The stone may become charged with static when warmed, attracting dust or paper.

Look-alikes depend on color: green tourmaline can resemble emerald or peridot, pink resembles ruby or pink sapphire, blue resembles aquamarine. Tourmaline's strong dichroism, triangular cross-section and striations distinguish it from these.

Colorful tourmaline is widely used in jewelry, from affordable green and pink stones to highly prized Paraiba and fine rubellite. It is an October birthstone and an 8th-anniversary gem.

Because of its pyroelectric and piezoelectric properties, tourmaline has minor technical uses in pressure gauges and was historically used to polarize light. Powdered tourmaline is marketed in consumer products, though many such claims are unsupported.

Metaphysically, black tourmaline is popular for protection and grounding, and colored varieties are associated with various energies, though these uses are not scientific.