In the gemstone industry, the Tourmaline Group is recognized as one of the most chemically complex mineral species available. It is not a single mineral but a broad group of borosilicates that share a common crystal structure while possessing widely varying chemical compositions. This internal variety allows Tourmaline to occur in a broader range of colors than almost any other gemstone family.
1. Chemical Complexity and Classification The general chemical formula for the Tourmaline group is incredibly broad: $XY_3Z_6(T_6O_{18})(BO_3)_3V_3W$. In this formula, the variables represent different sites in the atomic lattice that can be occupied by elements such as Sodium, Lithium, Aluminum, Iron, Magnesium, or Copper. The Major Species: While there are over 30 recognized species of Tourmaline, the trade focuses on four primary categories: Elbaite: The most common source of gem-quality stones. It is lithium-rich and produces the famous greens, blues, and pinks. Schorl: Iron-rich and typically black. While historically used for mourning jewelry, it is primarily used today for industrial purposes and ornamental carvings. Dravite: Magnesium-rich, producing earthy tones ranging from deep yellow to dark brown. Liddicoatite: Calcium-rich and famous for its complex color zoning, often seen in sliced sections.
2. Optical and Physical Phenomena Tourmaline possesses two specific physical properties that are critical for both industrial use and gemstone identification: I. Pleochroism Tourmaline is strongly pleochroic, meaning it shows different colors or depths of color when viewed from different angles (specifically along the “c-axis” versus the “a-axis”). Practical Impact: A master cutter must orient the crystal correctly. If cut poorly, a green tourmaline may appear overly dark or “dead” because the light is being absorbed along the darker axis of the crystal. II. Pyroelectricity and Piezoelectricity When heated (pyroelectric) or put under mechanical pressure (piezoelectric), a tourmaline crystal develops an electric charge. The “Ash-Drawer”: Historically, Dutch traders called it Aschentrekker because it would attract ash from their pipes. On a modern jewelry display, this means tourmaline attracts more dust than other gems and requires frequent cleaning.
3. Key Commercial Varieties In the professional market, tourmalines are often sold by descriptive trade names based on their chemical “accidents”: Paraíba Tourmaline: Colored by Copper and Manganese. These are the most expensive tourmalines, prized for their “electric” neon-blue or turquoise hues. Originally found in Brazil, significant deposits have since been discovered in Nigeria and Mozambique. Rubellite: A specific grade of pink-to-red tourmaline. To be classified as Rubellite, the stone must maintain its red color under different lighting conditions (it should not shift to a “hot pink”). Indicolite: A rare, iron-bearing blue variety. Chrome Tourmaline: Colored by Chromium and Vanadium. This is a rare variety found in East Africa (Tanzania) that produces a green so vivid it rivals high-end emeralds. Watermelon Tourmaline: A bi-color or tri-color phenomenon where a pink core is surrounded by a green “rind,” separated by a white or colorless band.
4. Technical Specifications Property Technical Value Chemical Formula Complex Borosilicate Mohs Hardness 7.0 – 7.5 Crystal System Trigonal (Long, striated prisms) Refractive Index 1.624 – 1.644 Birefringence 0.018 – 0.040 Specific Gravity 3.06 (±0.20)
5. Geological Formation and Sourcing Tourmalines primarily form in pegmatites—pockets of igneous rock where slow cooling allows large, high-clarity crystals to grow. They are often found in association with quartz and feldspar. For a “Mine-to-Market” operation, tourmaline is a high-value asset because it is often found in large crystals, allowing for the selection of “clean” sections for high-carat faceting. East African mines are currently the primary source for “Chrome” and “Paraíba-type” (copper-bearing) materials, making direct access to these strikes a significant competitive advantage.
