Tourmaline is a crystalline boron silicate mineral group, known for its remarkable diversity in color. This gemstone displays almost every color of the rainbow, from colorless to black, and even multiple colors within a single crystal. Its name, derived from the Sinhalese word “tōramalli,” means “mixed gems,” aptly describing its varied appearance. Tourmaline forms in diverse geological environments worldwide, making it a widespread mineral.
Global Presence
Tourmaline deposits are found across several continents, with significant occurrences in South America, Africa, Asia, and North America. Brazil is a major producer, especially in Minas Gerais and Bahia, yielding a wide spectrum of colors. Africa is another important source, with countries like Namibia, Nigeria, Mozambique, Madagascar, Tanzania, and Kenya contributing to the global supply.
Asia also hosts significant tourmaline deposits, particularly in Afghanistan and Pakistan, where fine pink, red, green, and blue specimens are found. Sri Lanka, historically known as Ceylon, has been a source for centuries, producing various colors including yellow and brown. In the United States, tourmaline has been mined in California and Maine, with these regions historically producing pink, green, and bi-colored varieties.
Geological Settings
Tourmaline commonly forms in specific geological environments, primarily in pegmatites. These are coarse-grained igneous rocks that crystallize during the final stages of magma cooling, often enriched with rare elements like boron, which is crucial for tourmaline formation. Pegmatites provide the necessary conditions for tourmaline crystals to grow to significant sizes and often produce gem-quality material. Lithium-rich pegmatites are particularly noted for yielding vibrantly colored tourmalines.
Beyond igneous settings, tourmaline is also found in metamorphic rocks, such as schists and gneisses. In these environments, tourmaline can form through the alteration of existing rocks under intense heat and pressure. The presence of boron-rich fluids during regional metamorphism facilitates the crystallization of tourmaline within these altered rock types. Tourmaline’s durability also allows it to persist and accumulate in alluvial deposits. These deposits form when tourmaline-bearing rocks erode, and the resistant mineral grains are transported by water and concentrated in riverbeds or ancient streambeds.
Natural Appearance
In its natural state, tourmaline typically appears as prismatic or columnar crystals, often exhibiting distinctive vertical striations along their length. A unique characteristic of tourmaline crystals is their triangular or sometimes six-sided cross-section with rounded edges. Within the rock matrix, these crystals can range from opaque to transparent, showcasing their potential gem quality.
Tourmaline’s wide color spectrum is often visible even in unmined crystals, with single specimens sometimes displaying multiple colors in distinct zones. This phenomenon, known as color-zoning, results from variations in fluid chemistry during the crystal’s growth. Common associated minerals found alongside tourmaline in pegmatites include quartz, feldspar, and mica.
Famous Localities
Brazil’s Minas Gerais state is renowned for its diverse tourmaline output, including vibrant greens, pinks, and purplish-red rubellites, as well as the distinctive watermelon tourmaline. The state of Paraíba, Brazil, became famous for its intensely vivid “neon” blue and green copper-bearing tourmalines, discovered in the late 1980s. These Paraíba tourmalines are highly prized for their unique electric glow, though similar material has since been found in Africa.
In the United States, California’s Pala and Mesa Grande districts, particularly the Himalaya Mine, have historically produced significant quantities of pink and green tourmaline. Maine, with localities like Mount Mica and Newry, has also been a consistent source of quality tourmaline since the early 19th century, yielding raspberry pink-red and minty green crystals. In Africa, the Usambara Mountains in Tanzania are known for producing chrome tourmaline, a rich green variety. Afghanistan’s Nuristan and Panjshir Valley regions are notable for their fine pink, red, and intensely green tourmalines, often with sharp color zoning.