- "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 is easy to find in jewelry stores. Well-formed tourmaline crystals are also valued by mineral specimen collectors. Specimens with attractive colors and crystal forms can sell for thousands of dollars.
Physical Properties of Tourmaline
- a prismatic crystal habit and often has obvious striations that parallel the long axis of a crystal.
- often have triangular or six-sided cross-sections with rounded edges.
- often color zoned through their cross-sections or along their length.
- can be pleochroic (crystal absorbs different wavelengths of light differently depending on the direction of incidence of the rays or their plane of polarization, often resulting in the appearance of different colors according to the direction of view) with the darkest color viewing down the C-axis and lighter color viewing perpendicular to the C-axis.
- has indistinct cleavage, so any specimen with obvious cleavage is probably not tourmaline. Color might not be helpful. The most common tourmaline color is black, but the mineral occurs in all colors of the spectrum.
|Black is the most common color. Also occurs in blue, green, yellow, pink, red, orange, purple, brown, and colorless. Single crystals are often zoned.
|White when softer than the streak plate. Colorless when harder than the streak plate.
|Transparent to translucent to nearly opaque
|7 to 7.5
|2.8 to 3.3
|Lack of visible cleavage, prismatic crystals with rounded triangular cross-sections that are often striated, vibrant colors, pleochroism.
Geologic Occurrence of Tourmaline
- Tourmaline most commonly occurs as an accessory mineral in igneous and metamorphic rocks. Large, well-formed crystals can form during hydrothermal activity when hot waters and vapors carry the elements needed to form tourmaline, into pockets, voids, and fractures. 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.
- 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. So, particles of tourmaline weathered from igneous or metamorphic rocks can persist in a stream and can be transported long distances from their source area.
- Tourmaline gem rough is mined from stream sediments in many parts of the world. It generally occurs as small granules and pebbles that that have been rounded by the abrasion of stream transport. Tourmaline is often one of many different minerals produced from a single mining location.
Alluvial tourmaline: About 30 carats of stream-rounded tourmaline rough from Tanzania in yellow, orange, and green colors.
Tourmaline as an Accessory Mineral
- The most common occurrence of tourmaline is as an accessory mineral in igneous and metamorphic rocks. It often occurs as millimeter-size crystals scattered through granite, pegmatite, and gneiss. In this mode of occurrence, tourmaline rarely makes up more than a few percent of the rock's volume. The variety of tourmaline most often found as an accessory mineral is black schorl.
Accessory tourmaline: A specimen of the Crabtree Pegmatite from North Carolina, showing black prismatic tourmaline and green emerald crystals in a matrix of white feldspar and quartz. The width of this view is about two inches.
- Brazil has been the world's leading source of tourmaline for nearly 500 years. In the 1500s Portuguese explorers obtained green and blue tourmaline from indigenous people and from panning streams in search for gold. They thought that these colorful stones were emeralds and sapphires and sent them back to Portugal where they were cut into gems. The misidentification of these gems was not discovered until over 100 years later. Since the late 1800s, a steady stream of tourmaline discoveries have been made in Brazil, and a diverse stream of tourmaline gem materials have supplied the gem and jewelry market. Today, discoveries of tourmaline of various kinds are made in Afghanistan, Mozambique, Namibia, Nigeria, Pakistan, Tanzania, the United States and other countries. These provide the market with a constantly-changing supply of gem tourmaline and mineral specimens.
- Tourmaline is a complex boron silicate mineral with a generalized chemical composition of:
- Letters in the formula above represent positions in the atomic structure of tourmaline that can be occupied by ions listed below.
- X = Ca, Na, K,  ( = vacancy)
- Y = Li, Mg, Fe+2, Fe+3, Mn+2, Al, Cr+3, V+3
- Z = Mg, Al, Fe+3, V+3, Cr+3
- T = Si, Al, B
- V = OH, O
- W = OH, F, O
- The complex formula and many substituting ions produce the large number of minerals in the tourmaline group. The International Mineralogical Association has recognized 32 different tourmaline minerals based upon the chemical composition of solid solution series end members.
The most common species of tourmaline is schorl. It may account for 95% or more of all tourmaline in nature. The early history of the mineral schorl shows that the name "schorl" was in use prior to 1400 because of a village named "Schorl" in Germany where the black tourmaline was found in a tin mine. It is usually dark brown to black.
Pakistan John A. Betts
Dravite: It is usually dark yellow to brownish black. Power's Farm, Pierrepont, St. Lawrence Co., New York, USA
Maggie Wilson, P. Nicholson
Elbaite species: a sodium, lithium, aluminium boro-silicate, named after the island of Elba, Italy Red or pinkish-red—rubellite variety, Brazil, Knut Eldjarn
Elbaite species Green—verdelite or Brazilian emerald variety, Brazil Antonio Borrelli
Elbaite species, Colorless—achroite variety (from the Greek meaning "colorless"). Afghanistan Rob Lavinsky & irocks.com
Elbaite species Light blue to bluish green—Brazilian indicolite variety (from indigo), Namibia Stephan Koch
Six faceted tourmaline gemstones from Africa. Clockwise from top left: A blue-green oval weighing 5.5 carats; emerald-cut chrome tourmaline, 1.51 carats; green round, 1.87 carats; pink emerald cut, 1.04 carats; pink-orange emerald cut, 1.88 carats; red cushion cut, 3.34 carats. (Photos are not to scale.) Specimens and images copyright by Lapigems.
Names Used for Tourmaline Gems
- Because it can be impossible or impractical to determine the chemical composition of a large number of specimens or even a single specimen, the generic name "tourmaline" is typically used for any mineral in the tourmaline group in the field, the classroom, the office, or even in a laboratory.
- Jewelers and gemologists use trade names for different colors of tourmaline to simplify communications with their customers. These names work much better in a jewelry store than the mineralogical names .
- 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".
- For other tourmaline colors, the name of the color is used as an adjective. For example, "pink tourmaline" or "purple tourmaline." "Yellow tourmaline" is sometimes sold as "canary tourmaline".
Color Zoning in Tourmaline
- Changing conditions during tourmaline crystal growth often result in single crystals that contain two or more different colors of tourmaline. These bicolor crystals are known as "zoned crystals." Cut gemstones with distinctly different color zones are known as parti-color gems.
- 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. 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.
Watermelon tourmaline: rough (left)and faceted (right) tourmalines that exhibit a superb example of watermelon color. Both have clarity problems. This is typical for bicolor tourmaline. Specimens with perfect clarity near the color transition are extremely rare. The change in conditions that caused the color change might have also disrupted the crystal growth to produce the clarity problems. From Minas Gerais, Brazil. The rough crystal measures approximately 4.2 x 1.4 x 1.1 cm, and the faceted gem measures 27.79 mm x 18.51 mm and weighs nearly 50 carats.
Specimens and photos by Arkenstone/www.iRocks.com.
Tourmaline crystal cross-section: A "slice" of watermelon tourmaline which shows the pink interior, green outer layer, and triangular shape of the crystal. This specimen shows the origin of the "watermelon" name.
Image copyright iStockphoto/Sun Chan.
- Tourmaline is one of many minerals that can be chatoyant when cut into a gem. "Chatoyant" is a gemological adjective used to describe minerals that exhibit a "cat's-eye". Chatoyant tourmalines contain thousands of tiny parallel tubes that have the ability to reflect light. When a tourmaline crystal filled with these tubes is properly cut into a cabochon, a line of bright light known as a cat's-eye will be reflected from the dome of the cabochon. The proper orientation is obtained by cutting the cabochon with the tubes paralleling the base of the cabochon and crossing the long dimension of the cabochon at a right angle.
- Cat's-eye gems are fun to observe because the "eye" will move back and forth across the dome of the stone in three situations:
1) when the stone is moved under the light,
2) when the source of light is
3) when the head of the observer is moved.
Cat's-Eye Tourmaline: If you look closely you can see the tiny tubes running from left to right within the gem. This wonderful tourmaline is owned by the Edelsteinmuseum (Gemstone Museum) in Idar-Oberstein, Germany and the photo was taken by a photographer who works under the name of Vassil and places many of his photos in the public domain.
- Heat and irradiation are common treatments used to improve the color of tourmaline. Both of these treatments are commonly done after the stones have been cut and polished. They can be undetectable when viewed with a gemological microscope.
- Heat treatment can lighten an undesirable tone in some materials and give some brownish stones a brighter, more desirable color. The results of heat treatment are usually permanent. Stones with liquid inclusions are not good candidates for heat treatment because heating can cause them to fracture.
- Irradiation treatment can brighten many light-colored stones. The results are often reversed if the stones are heated. They can also be reversed over time with exposure to bright light.
- Imitation tourmaline is occasionally seen. The popular watermelon tourmaline and other parti-colored tourmalines are a common target of the imitators. Some assembled imitation stones consist of a thin wafer of colored glass or plastic, glued between two pieces of colorless glass.
- These imitations are easy to detect with a microscope or loupe. If the stones are examined along the girdle, the edge of the colored wafer or the glue line can usually be seen. If the stones are examined by looking down through the table, bubbles or debris in the glue plane are sometimes visible.
Imitation Tourmaline: An assembled-stone imitation of watermelon tourmaline. It consists of a thin wafer of colored plastic that is glued between two pieces of clear glass and then faceted.