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Tourmaline


Earth's most colorful mineral and gem material.


Author: , Ph.D., GIA Graduate Gemologist


tourmaline crystals

Colorful tourmaline crystals: A couple dozen small tourmaline crystals in pretty colors from Afghanistan, suitable for faceting very small stones. Some of them are bicolor and a few are oriented to show pleochroism, with the color looking down the long axis of the crystal being much darker than when looking at the crystal in side view.

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
    Crystals in Fractures, Voids, Pockets
    Alluvial Tourmaline
    Tourmaline as an Accessory Mineral
Tourmaline Sources
Physical Properties of Tourmaline
Tourmaline Chemistry
Names Used for Tourmaline Gems
"Paraiba" - The Problem Name
Color Zoning in Tourmaline
Cat's-Eye Tourmaline
Pleochroism in Tourmaline
Tourmaline Treatments
Synthetic Tourmaline
Imitation Tourmaline


tourmaline crystals on cleavelandite

Tourmaline crystals on cleavelandite: A large mineral specimen consisting of prismatic tourmaline crystals on cleavelandite with quartz and lepidolite. The tourmaline crystals are color zoned with red tourmaline at the base that sharply transitions to blue-green along their length. From the Pederneira Mine of Minas Gerais, Brazil. Measures 21 x 15 x 14 cm. Specimen and photo by Arkenstone / www.iRocks.com.

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 rough

Alluvial tourmaline: About 30 carats of stream-rounded tourmaline rough from Tanzania in yellow, orange, and green colors.

Alluvial Tourmaline


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 in matrix

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.

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.



Tourmaline Sources

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. [1]

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.
Luster Vitreous
Diaphaneity Transparent to translucent to nearly opaque
Cleavage Indistinct
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)
Crystal System Hexagonal
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.

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

Tourmaline: 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.

Tourmaline Chemistry

Tourmaline is a complex boron silicate mineral with a generalized chemical composition of:

XY3Z6(T6O18)(BO3)3V3W

Letters in the formula above represent positions in the atomic structure of tourmaline that can be occupied by ions listed below.

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 crystals

Tourmaline crystals: Tourmaline crystals often have many fractures and inclusions, but these crystals exhibit wonderful clarity and very rich color. They also show the striations along the long axis of the crystals that are characteristic of tourmaline. The blue-green cluster on the left sits atop cleavelandite with purple lepidolite, and it measures 13 cm tall. The rubellite cluster on the right measures 6.7 cm tall. Specimens and photos by Arkenstone / www.iRocks.com.

Mineral collection

The best way to learn about minerals is to study with a collection of small specimens that you can handle, examine, and observe their properties. Inexpensive mineral collections are available in the Geology.com Store.

Tourmaline Group Minerals

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MineralComposition
AdachiiteCaFe3Al6(Si5AlO18)(BO3)3(OH)3OH
BosiiteNaFe3(Al4Mg2)Si6O18(BO3)3(OH)3O
Chromium-draviteNaMg3Cr6Si6O18(BO3)3(OH)3OH
Chromo-alumino-povondraiteNaCr3(Al4Mg2)Si6O18(BO3)3(OH)3O
DarrellhenryiteNaLiAl2Al6Si6O18(BO3)3(OH)3O
DraviteNaMg3Al6Si6O18(BO3)3(OH)3OH
ElbaiteNa2(Li3,Al3)Al12Si12O36(BO3)6(OH)6(OH)2
FeruviteCaFe3(MgAl5)Si6O18(BO3)3(OH)3OH
Fluor-buergeriteNaFe3Al6Si6O18(BO3)3O3F
Fluor-draviteNaMg3Al6Si6O18(BO3)3(OH)3F
Fluor-elbaiteNa2(Li3,Al3)Al12Si12O36(BO3)6(OH)6F2
Fluor-liddicoatiteCa(Li2Al)Al6Si6O18(BO3)3(OH)3F
Fluor-schorlNaFe3Al6Si6O18(BO3)3(OH)3F
Fluor-tsilaisiteNaMn3Al6Si6O18(BO3)3(OH)3F
Fluor-uviteCaMg3(Al5Mg)Si6O18(BO3)3(OH)3F
Foitite[](Fe2Al)Al6Si6O18(BO3)3(OH)3OH
LucchesiiteCa(Fe)3Al6Si6O18(BO3)3(OH)3O
Luinaite-(OH)(Na,[])(Fe,Mg)3Al6Si6O18(BO3)3(OH)3OH
Magnesio-foitite[](Mg2Al)Al6Si6O18(BO3)3(OH)3OH
MaruyamaiteK(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O
OleniteNaAl3Al6Si6O18(BO3)3O3OH
Oxy-chromium-draviteNaCr3(Mg2Cr4)Si6O18(BO3)3(OH)3O
Oxy-draviteNa(Al2Mg)(Al5Mg)Si6O18(BO3)3(OH)3O
Oxy-schorlNa(Fe2Al)Al6Si6O18(BO3)3(OH)3O
Oxy-vanadium-draviteNaV3(V4Mg2)Si6O18(BO3)3(OH)3O
PovondraiteNaFe3(Fe4Mg2)Si6O18(BO3)3(OH)3O
Rossmanite[](LiAl2)Al6Si6O18(BO3)3(OH)3OH
SchorlNaFe3Al6Si6O18(BO3)3(OH)3OH
TsilaisiteNaMn3Al6Si6O18(BO3)3(OH)3OH
UviteCaMg3(Al5Mg)Si6O18(BO3)3(OH)3OH
Vanadio-oxy-chromium-draviteNaV3(Cr4Mg2)Si6O18(BO3)3(OH)3O
Vanadio-oxy-draviteNaV3(Al4Mg2)Si6O18(BO3)3(OH)3O
tourmaline gemstones

Faceted tourmaline: A collection of faceted tourmalines of various colors. Some of these stones exhibit multiple colors because they were cut from color-zoned crystals. Two are pink and green bicolor stones known as "watermelon tourmaline." Bicolor and pleochroic tourmalines are favorite stones of many jewelry designers because they can be used to make especially interesting pieces of jewelry. The small round stones weigh about 0.5 carat each. The watermelon in the lower left corner weighs 0.61 carat.

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!

"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 Tourmaline

Cuprian Tourmaline: A small crystal of the famous "Paraiba" tourmaline from the Batalha Mine in the state of Paraiba, Brazil. The spectacular blue color is caused by trace amounts of copper in the mineral. This specimen measures 1.5 x 1.3 x 0.9 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

"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.

watermelon tourmaline

Watermelon tourmaline: A pair of rough and faceted tourmalines that exhibit a superb example of watermelon color. Both the faceted stone and the crystal 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.

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.

tourmaline crystal cross-section

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.

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.

Cat's-Eye Tourmaline

Cat's-Eye Tourmaline: This specimen of tourmaline contains thousands of tiny reflective tubes. If you look closely you can see them running from left to right within the gem. When a beam of light from above strikes the gem, the observer sees a bright line of light reflecting from the tubes below the surface of the gem. This bright line is known as a "cat's-eye" and this gem is known as "cat's-eye tourmaline". 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.

Cat's-Eye Tourmaline

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.

Video: Pleochroism in Tourmaline: This video demonstrates pleochroism in two short sections of tourmaline crystals. The crystal sections are lightest when viewed perpendicular to the c-axis of the crystal (the side being held by the gem tweezers) and they are darkest when viewed down the c-axis of the crystal.

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.

Video: Pleochroism in Tourmaline: This video demonstrates pleochroism in two short sections of tourmaline crystals. The crystal sections are lightest when viewed perpendicular to the c-axis of the crystal (the side being held by the gem tweezers) and they are darkest when viewed down the c-axis of the crystal.

Tourmaline Treatments

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.

Tourmaline Information
[1] 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.

[2] 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.

[3] 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

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. [2] [3]

Imitation Tourmaline

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.

Imitation 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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