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Earth's most colorful mineral and gem material.

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.

What is Tourmaline?

"Tourmaline" is the name of a large group of boron silicate minerals that share a common crystal structure and similar physical properties - but vary tremendously in chemical composition.

The wide range of compositions, along with trace elements and color centers, causes tourmalines to occur in more colors and color combinations than any other mineral group. Crystals of good color and clarity are often cut into beautiful gemstones. Tourmaline is such a popular gemstone that it is easy to find in jewelry stores. Nice tourmaline crystals are also valued by mineral specimen collectors. Specimens with attractive colors and habits can sell for thousands of dollars.

Tourmalines commonly occur as accessory minerals in igneous and metamorphic rocks. Large crystals of tourmaline can form in cavities and fractures during hydrothermal activity. Tourmaline also exists as durable grains in sediments and sedimentary rocks.

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.

Geologic Occurrence of Tourmaline

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 mineral most often found as an accessory mineral is black schorl.

tourmaline crystals on cleavelandite

Tourmaline crystals on cleavelandite: A large mineral specimen consisting of prismatic tourmaline crystals on cleavelandite with quartz and lepidolite. From the Pederneira Mine of Minas Gerais, Brazil. Measures 21 x 15 x 14 cm. Specimen and photo by Arkenstone / www.iRocks.com.

Crystals in Fractures Voids and Pockets

The most spectacular tourmaline crystals are formed by hydrothermal activity. They are found in pockets, voids, or fractures and range in size from tiny millimeter crystals to massive prisms weighing over 100 kilograms. A rich pocket of nice tourmaline crystals can yield mineral specimens and gem materials worth millions of dollars.

tourmaline rough

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

Alluvial Tourmaline

With a hardness of 7 to 7 1/2, tourmaline weathered from igneous or metamorphic rocks can be a durable sediment grain. Tourmaline gem rough is mined from streams sediments in many parts of the world, often by artisanal miners. Tourmaline is often one of many minerals produced from a single mining location.

watermelon tourmaline

Watermelon tourmaline: A set of rough and cut tourmaline that shows superb color zoning in "watermelon" colors. 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.

Physical Properties of Tourmaline

Chemical Classification Boron silicate
Color Usually black. 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
Chemical Composition (Ca,Na,K,[]) (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. 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. And, 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 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 crystals

Colorful tourmaline crystals: A couple dozen small tourmaline crystals of various colors from Afghanistan, suitable for faceting small stones.

tourmaline crystals

Tourmaline crystals: Tourmaline typically has many fractures and inclusions, but these crystals exhibit wonderful clarity. 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 Chemistry

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.

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

Tourmaline cabs

Tourmaline cabs: Numerous cabochons of tourmaline from Africa. Each gem is approximately 6 millimeters by 4 millimeters.

Tourmaline beads

Tourmaline beads: The variety in tourmaline color is shown with this strand of beads.

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. Image © iStockphoto / Sun Chan.

More Minerals
  Mohs Hardness Scale
  The Acid Test
  Rock, Mineral and Fossil Collections.

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