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Tourmaline is the name of a large group of boron silicate minerals. These minerals share a common crystal structure and similar physical properties – but vary tremendously in chemical composition. The wide range of compositions and color zoning within crystals causes tourmaline to occur in more colors and color combinations than any other mineral group.
Tourmaline is one of the world’s most popular gemstones and it serves as a birthstone for the month of October. Because of its popularity, tourmaline is easy to find in jewelry stores. Well-formed tourmaline crystals are also valued by mineral specimen collectors. Pink and green stones are the most popular. Some crystals are multicolored with different colors occurring at either end of the crystal or forming a core and rim such as watermelon tourmaline.
Tourmaline most commonly occurs as an accessory mineral in igneous and metamorphic rocks. Large, well-formed crystals of tourmaline can form in cavities and fractures during hydrothermal activity. Tourmaline is a hard and tenacious mineral. That enables it to persist during stream and beach transport as durable grains in sediments and sedimentary rocks.
The most spectacular tourmaline crystals are formed by hydrothermal activity. These crystals form when hot waters and vapors carry the elements needed to form tourmaline into pockets, voids, and fractures, which offer an open space for crystal growth. The tourmaline crystals formed in these cavities range in size from tiny millimeter crystals to massive prisms weighing over 100 kilograms. They present as elongates crystals with distinctive “rounded triangular” shape in cross section.
Tourmaline has a Mohs hardness of 7 to 7 ½, and that hardness makes it a durable sediment granule. Tourmaline is also relatively resistant to chemical weathering.
Tourmaline has a few properties that can aid in its identification. If you have a tourmaline crystal, identification should be easy.
Red tourmaline is sold as “rubellite”.
Blue tourmaline is sold as “indicolite”.
Green tourmaline colored by chromium or vanadium is often sold as “chrome tourmaline”.
Black tourmaline is sold as “schorl”.
Dravite species such as green chromium dravite and brown dravite are diamagnetic.
For other tourmaline colors, the name of the color is usually used as an adjective. For example, “pink tourmaline” or “purple tourmaline.” “Yellow tourmaline” is sometimes sold as “canary tourmaline”.
In many gems, color zoning is undesirable because most gem and jewelry buyers prefer stones that have a single, uniform face-up color. Tourmaline is an exception to this trend. Gems cut from color-zoned crystals with pleasing colors are a novelty prized by designers and collectors.
Color-zoned crystals are often sawn into thin cross-sections and polished. These thin bicolor gems can be very attractive. The most popular bicolor tourmaline is “watermelon tourmaline.” It has a pink interior and a green rind – just like a slice of watermelon. The closer the colors match those of a real watermelon, the more people enjoy them and the higher the price.
