Tourmaline is a gemstone known for its diversity and spectrum of colors. Its unique properties and broad palette of hues make it widely appealing.
Defining Characteristics
Tourmaline is a complex crystalline boron silicate mineral whose chemical composition varies due to elements such as aluminum, iron, magnesium, sodium, lithium, or potassium. This variability contributes significantly to its wide range of appearances. The mineral crystallizes in the trigonal system, typically forming elongated prismatic crystals that often exhibit a distinct triangular cross-section and parallel striations.
The hardness of tourmaline ranges from 7 to 7.5 on the Mohs scale, making it a durable gemstone. Its specific gravity generally falls between 2.8 and 3.3, a value that can fluctuate based on its precise chemical makeup. The refractive index of tourmaline is typically between 1.61 and 1.68, influencing how light interacts with the stone.
Tourmaline exhibits strong pleochroism, displaying different colors or intensities when viewed from varying angles. Beyond its visual attributes, tourmaline exhibits distinct electrical properties. It is pyroelectric, generating an electrical charge when heated, and piezoelectric, generating a charge when subjected to pressure. These electrical characteristics arise from its asymmetric crystal structure.
Spectrum of Varieties
The broad spectrum of colors in tourmaline results from various trace elements within its complex structure. For instance, iron can produce green, blue, and black hues, while manganese typically yields pink, red, and yellow shades. Chromium and vanadium are responsible for intense green varieties, and copper imparts vivid neon blue and green colors.
Specific trade names are associated with these color variations. Rubellite refers to red or pink tourmaline, while Indicolite describes blue varieties. Green tourmaline is often called Verdelite, and the most common variety, black tourmaline, is known as Schorl. Dravite is a brown to yellowish-brown tourmaline, and Achroite is a rare, colorless variety.
Unique formations include Watermelon Tourmaline, which features a pink core surrounded by a green outer rind, resembling the fruit. Paraiba Tourmaline is highly sought-after for its neon blue, green, or violet colors caused by the presence of copper and manganese. Although originally discovered in Brazil, similar copper-bearing tourmalines have also been found in Nigeria and Mozambique.
Geological Origins and Occurrence
Tourmaline forms in diverse geological settings, primarily crystallizing within igneous and metamorphic rocks. Gem-quality tourmaline often originates in pegmatites, coarse-grained igneous rocks that form during the final stages of magma cooling. During this process, incompatible elements like boron and lithium become concentrated in the residual melt, facilitating tourmaline’s growth.
The mineral also forms in hydrothermal veins, where hot, mineral-rich fluids circulate through cracks and fissures, depositing tourmaline crystals as they cool. In metamorphic rocks, tourmaline can develop through the recrystallization of existing boron-containing sediments or via boron metasomatism, where boron-rich fluids alter the rock’s composition. These processes often lead to visible crystals.
Major global sources include Brazil, particularly Minas Gerais and Paraíba, known for their wide array of colors and the rare Paraiba variety. Other prominent mining locations are Afghanistan, Pakistan, and various African nations such as Nigeria, Mozambique, Namibia, Tanzania, and Kenya. The United States also has historical tourmaline mining regions in Maine and California.
Applications and Uses
Tourmaline is a popular choice in the jewelry industry for rings, necklaces, bracelets, and earrings, due to its beauty, durability, and extensive color range. Its ability to display multiple colors within a single crystal or exhibit strong pleochroism adds to its appeal in design.
Beyond its decorative uses, tourmaline has found applications in scientific and industrial fields due to its unique electrical properties. Its piezoelectricity, the ability to generate an electrical charge under pressure, has led to its use in pressure gauges and sensors. Tourmaline’s pyroelectric property, where it generates a charge when heated, has been leveraged in optical instruments and to control static in electronics manufacturing processes. Powdered tourmaline has also been explored as an ingredient in ceramic water purification systems.