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.
Geologic Occurrence of Tourmaline
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.
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.
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.
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.|
|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|
|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)|
|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 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
Names Used for Tourmaline Gems
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. 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." These names work much better in a jewelry store than the mineralogical names in the table above!
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."
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 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 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.
Some geologists use the gemological names for members of the tourmaline group. However, determining the correct mineralogical name can be impossible in the field or a classroom, so communication is done with these alternative names.
Color Zoning in Tourmaline
Changing conditions during tourmaline crystal growth often result in single crystals that contain two different colors of tourmaline. The earlier color is usually overgrown by the later color. These bicolor crystals are known as "zoned crystals."
In many gems, color zoning is undesirable. 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 sawn and 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 will not kill their desirability or price.
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.
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?
|Rock, Mineral and Fossil Collections.|
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