G&G in a Flash: Micro-Features of Tourmaline
November 22, 2024
Tourmaline is a mineral supergroup that includes gems with a wide range of chemical compositions. More than 40 recognized tourmaline species exist in all the colors of the rainbow. Tourmalines generally form elongated crystals with triangular cross sections, and they are often color zoned. This color zoning results from changes in the mineral solution when the tourmaline crystallized.
Treatment
Tourmaline is unique in that it has no laboratory-grown counterpart. Any tourmaline you see on the market (if it is indeed tourmaline) is natural. Tourmalines are occasionally treated, with pink tourmalines irradiated to increase their color saturation, and blue and green stones undergoing low-temperature heat treatment to improve or lighten their colors. Such treatments are often undetectable and don’t typically affect a tourmaline’s inclusions. Fracture filling is generally the only treatment that leaves any visible evidence.
This 29.34 ct Paraíba tourmaline from Mozambique showcases the gem’s signature neon green to blue color. Courtesy: Granada Gallery
Sources
While tourmaline is very common as a mineral, gem-quality stones are rare. Most gem-quality tourmalines are the lithium-rich species elbaite, found in granitic pegmatites that have high amounts of boron and lithium. Notable sources are Brazil, the United States, Madagascar, Nigeria, and Mozambique. The most highly prized gem tourmalines are the vividly colored green to blue copper-bearing variety known in the trade as “Paraíba” tourmaline, named after the state in Brazil where it was first discovered. Today, these copper-bearing tourmalines are also found in Mozambique and Nigeria. “Paraíba-type” tourmalines are usually elbaite, but liddicoatite examples also exist.
This 6.93 ct tourmaline from Brazil has a cat’s-eye effect due to growth-tube inclusions.
Inclusions in Tourmaline and Tourmaline as Inclusions
Growth Tubes
Tourmalines often contain growth tubes caused by a localized interruption of crystal growth, leaving behind hollow or fluid-filled tubes. These features are sometimes called growth blockages. When enough of them are oriented properly in a cabochon, they can produce a cat’s-eye effect.
Mineral Inclusions
Solid mineral inclusions reflecting the growth environment are also found in gem tourmaline. Elbaite gems that formed in pegmatites may contain minerals such as lepidolite, feldspar, zircon, or pyrochlore. Tourmalines from metamorphic deposits may contain other minerals such as graphite or apatite. Tourmaline inclusions are sometimes seen in pegmatite minerals, including beryl, quartz, mica, and spodumene.
Fluid Inclusions
Gem tourmalines often contain fluid inclusions; they are called “trichites” when they show a thready or hairlike structure. Some of these may be three-phase inclusions, containing liquid, gas, and solid components. When tourmalines containing fluid inclusions are heat treated, the fluid inclusions tend to rupture, leaving fractures. These fractures can be made less visible through filling with oils or resins.
For a close-up look at these inclusions in colorful photomicrographs, read the full article.
