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A Guide to Buying Tourmaline Tourmaline is the general name used for a group of related mineral species. It was named after the Sinhalese word “tourmali” which means mixed parcel. Tourmaline is a pegmatite mineral and comes mostly from
Color is by far the most important attribute in determining value. Tourmaline comes not only in every color imaginable, but often you will find specimens with multiple colors in the same stone. Some of the more special varieties include: · Watermelon – Bi-color with a pink or red core and green outer shell · Paraiba – Intense blue to green from Paraiba, Brazil, colored by copper · Canary – Bright yellow tourmaline from Malawi · Chrome – Intense green colored by chromium or vanadium · Indicolite – Blue · Rubellite – Red · Color-Change – Changes from green in daylight to red in incandescent light · Cat’s Eye – Chatoyant tourmaline in a variety of colors As with all colored gems the color should be vivid and never too dark or too light. The best light for viewing tourmaline depends upon the color of the stone, with the “cooler colors” (green, blue, purple) appearing best in daylight, and the “warmer colors” (red, orange, yellow) appearing better in incandescent light. Tourmaline is also well known for being strongly pleochroic. The clarity of tourmaline depends largely on the color of the stone. Blue and green stones are usually very clean, where red and pink tourmalines commonly have eye-visible inclusions such as needles, fractures and liquid filled healed fractures. Heat treatments, irradiation and occasionally oiling are sometimes used to enhance the stone or hide fractures. You will find tourmaline in as many different cuts as you find colors in both cabochons and faceted stones. Due to the elongated nature of the crystals they are commonly cut into emerald and rectangle shapes to emphasize the lighter of the pleochroic colors on dark stones, and into rounds, ovals, triangles and trillions to emphasize the darker colors on lighter stones. In terms of size, chrome and rubellite tourmalines are rare above 10 carats, Paraiba tourmalines are rare above 2 carats and world-class above 5. As with all gems the price varies greatly depending on quality and variety; Paraiba tourmalines are the most expensive with some reaching the tens of thousands per carat range. For fine chrome, rubellite, indicolite and bi-color tourmalines the per carat price can easily be $1,000 or more, and other varieties in the $50 - $750 per carat range depending on the quality and color.
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Composition | Tourmaline is one of the most complex of all mineral groups, and includes the following species:
Tourmaline is a complex aluminum boro-silicate, with heavy emphasis on the “complex.” One pundit likened it more to a medieval alchemist’s brew than a respectable mineral species, and a glance at the above formulae would bear that out. |
Refractive Index | 1.624–1.644 (0.18–0.40; usually 0.20, may be greater in dark stones); doubly refractive, uniaxial negative |
Specific Gravity | 3.06 (+ 0.20; - 0.06) |
Hardness (Mohs) | 7 to 7.5 |
Crystal System | Hexagonal-trigonal |
Pleochroism | Strongly dichroic with the ordinary ray having a darker color |
Phenomena | Cat's eye tourmalines are common. Color-change chrome tourmalines, which change from green to red, are occasionally found. |
Dispersion | 0.017 |
Colors | Any and all. Tourmaline occurs in more colors than any other gem. Some colors have specific variety names, including:
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A variety of enhancements are regularly applied to tourmaline, depending on the source and variety. These include heat, irradiation, and oiling. | |
Handling | Ultrasonic: generally safe, but risky if the gem contains liquid inclusions Steamer: not safe The best way to care for tourmaline is to clean it with warm, soapy water. Avoid exposing it to heat or acids. |
Synthetic available? | No |
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