Earth's most colorful mineral and gem material.
Author: Hobart M. King, Ph.D., GIA Graduate Gemologist
What is Tourmaline?
"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.
Table of Contents
Geologic Occurrence of 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.
Tourmaline Crystals in Fractures, Voids, Pockets
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.
One rich pocket of nice tourmaline crystals can yield mineral specimens and gem materials worth millions of dollars. Many mineral collectors and gem hunters have become wealthy by discovering just one of these treasure-filled cavities.
Tourmaline has a Mohs hardness of 7 to 7 ½, and that hardness makes it a durable sediment grain. 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, often by artisanal miners. 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.
Tourmaline as an Accessory Mineral
The most common occurrence of tourmaline is as an accessory mineral in igneous and metamorphic rocks. It usually 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.
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. 
The first commercial gemstone mine in the United States followed an 1821 discovery of tourmaline near the town of Paris, Maine. Over the past 200 years, significant amounts of tourmaline have been produced in Maine, California, and other states from periodic discoveries.
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.
Physical Properties of Tourmaline
|Chemical Classification||Boron silicate|
|Color||Black is the most common color. Also occurs in blue, green, yellow, pink, red, orange, purple, brown, and colorless. Single crystals are often zoned.|
|Streak||White when softer than the streak plate. Colorless when harder than the streak plate.|
|Diaphaneity||Transparent to translucent to nearly opaque|
|Mohs Hardness||7 to 7.5|
|Specific Gravity||2.8 to 3.3|
|Diagnostic Properties||Lack of visible cleavage, prismatic crystals with rounded triangular cross-sections that are often striated, vibrant colors, pleochroism.|
|Chemical Composition||(Ca,Na,K,[vacancy]) (Li,Mg,Fe+2,Fe+3,Mn+2,Al,Cr+3,V+3)3 (Mg,Al,Fe+3,V+3,Cr+3)6 ((Si,Al,B)6O18) (BO3)3 (OH,O)3 (OH,F,O)|
|Uses||A popular gemstone and mineral specimen|
Physical Properties of Tourmaline
Tourmaline has a few properties that can aid in its identification. If you have a tourmaline crystal, identification should be easy.
- Tourmaline crystals are prismatic and often have obvious striations that parallel their long axis.
- They often have triangular or six-sided cross-sections with rounded edges.
- They are often color zoned through their cross-sections or along their length.
- Tourmaline is pleochroic with the darkest color viewing down the C-axis and lighter color viewing perpendicular to the C-axis.
Don't despair if your suspected tourmaline is an accessory mineral in an igneous or metamorphic rock. It often occurs in these rocks as tiny prismatic crystals. Get a hand lens and look for striations and rounded cross-sections.
Tourmaline 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.
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. These minerals are listed in the table below.
Tourmaline Group Minerals
| = vacancy|
Names Used for Tourmaline Gems
A table above lists the names and chemical compositions for 32 different members of the tourmaline mineral group. These names are based upon the chemical composition of the mineral. 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.
Tourmaline is one of the most popular gemstones because it occurs in every color of the spectrum. 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 in the table above!
- Dark red tourmaline is sold as "rubellite".
- Dark blue tourmaline is sold as "indicolite".
- Dark green tourmaline is 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 names" makes the language of tourmaline gems simple for jewelry consumers. If jewelry stores sold tourmaline gems by their scientific names, the chemical composition of each gem would need to be determined to assure that the names used to offer them for sale were absolutely accurate. This would create confusion, waste time and be an enormous expense. Tourmaline would not be as popular!
"Paraiba" - The Problem Name
Sometimes gem dealers inadvertently introduce trade names that result in confusion. In 1990, spectacular electric-blue tourmaline, colored by trace amounts of copper, was found at the Batalha Mine in the state of Paraiba, Brazil. The material was named "paraiba" after its locality. The beautiful gems were soon selling for over $2000 per carat, and the name "paraiba" rapidly spread through gemstone markets. People loved the gems, their colors, and their exotic name.
The name caused two problems in the gem trade. First, the name "paraiba" could be used when selling copper-bearing gems with the electric-blue color. It could also be used when selling any tourmaline from the state of Paraiba. "Paraiba Tourmaline" and "tourmaline from Paraiba" have different meanings to knowledgeable people. People who have heard the name "paraiba" but don't understand could be misled.
The second problem began in 2001 when electric-blue tourmaline was discovered in Nigeria. More was discovered in Mozambique in 2005. All of these tourmalines were marketed using the popular "paraiba" name. This confusion persists in the marketplace. It was slightly reduced when some sellers began using "African Paraiba" for gems mined in Nigeria and Mozambique.
Color Zoning in Tourmaline
Changing conditions during tourmaline crystal growth often result in single crystals that contain two or more different colors of tourmaline. The earlier color is usually overgrown by the later color. 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. 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.
Tourmaline crystals are also faceted to produce bicolor gems. "Watermelon" is again the most popular, but many other beautiful color combinations are cut.
Zoned tourmaline crystals often have clarity problems in the color-change area. If the color combination is attractive, minor clarity problems usually do not have a major impact on their desirability or price.
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.
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 moved, and 3) when the head of the observer is moved.
Please see our article about Chatoyant Gems.
Pleochroism in Tourmaline
Tourmaline is a pleochroic mineral. That means its apparent color can change with different directions of observation. The color is usually darkest looking down the c-axis of the crystal (down the long axis). It is usually lightest when viewing perpendicular to the long axis of the crystal.
Cutting pleochroic gem materials requires skill and knowledge. Rough must be studied and oriented to produce a gem with pleasing face-up color. A light piece of rough can be cut with the table of a stone perpendicular to the c-axis of the rough to maximize color. Dark rough can produce lighter gems if it is cut with the table plane of the stone parallel to the c-axis of the rough. Some rough can be cut to nicely display two pleochroic colors in the face-up position. Many jewelry buyers enjoy these gems.
Color optimization of pleochroic rough is time-consuming, requires special skills, and usually involves sacrifice. Which will produce a higher profit? A stone of premium color with a lower carat weight, or a larger stone with a less desirable color? These are the economics of faceting tourmaline.
Heat and irradiation are common treatments used to improve the color of tourmaline. Both of these treatments are done after the stones have been cut and polished. They are 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.
 Gem Pegmatites of Minas Gerais, Brazil: Exploration, Occurrence, and Aquamarine Deposits, by Keith Proctor, Gemological Institute of America, Gems & Gemology, Volume 20, Number 2, Summer 1984.
 Optical Absorption Spectroscopy of Synthetic Tourmalines, by M.N. Taran, A.S. Lebedev and A.N. Platonov; Physics and Chemistry of Minerals, Volume 20, Issue 3, pp.209-220, 1993.
 A Report on Synthetic Tourmalines in the Current Market, by Robert James; article published on the BadGemology.com website, 2008, accessed April 2018.
Synthetic tourmaline was being produced as early as 1991, but the first publication describing synthetic tourmaline found in the gem and jewelry market did not occur until 2008. These tourmalines were thought to be produced by hydrothermal growth with color being induced by lattice diffusion. The specimens were nearly flawless, and the best indication of their synthetic origin was a Raman shift that was unlike any natural tourmaline.  
Imitation tourmaline is occasionally seen. The popular watermelon tourmaline and other parti-colored tourmalines are a common target of the imitators. The assembled imitation stones often consist of a thin wafer of colored glass or plastic, glued between two pieces of colored 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 in the glue plane are sometimes visible.
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