Geological Properties Of Tourmaline
Geological Properties 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.
The Tourmaline Mineral Group
Tourmaline is not a single mineral, but a group of several closely related minerals. The three most well-known members are Elbaite, Schorl, and Dravite.
Tourmaline is extremely popular among collectors and is a well-known gemstone. It is the most multicolored mineral type known, occurring in virtually every color of the spectrum. Individual stones are often multicolored and are unsurpassed in their beauty.
The color of some Tourmaline can be enhanced through heat treatment. Some greenish stones can be made deep green, some brownish-red stones can be made red, and some light pink stones can be made colorless through heating.
Tourmaline has many interesting optical properties. Many green and blue specimens are strongly pleochroic. When viewed through their vertical axis, such specimens appear darker in color than when seen through their horizontal axis. In other Tourmalines, the color may actually be different when viewed at different angles because of the pleochroism. Certain Tourmalines exhibit a cat’s eye effect when polished into cabochons.
Tourmaline is both pyroelectric and piezoelectric. If a specimen is put under a pressure or temperature change, it will generate an electrical charge. When this happens, dust particles become attached to the crystal ends.
Tourmaline Crystals
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.
Geological Properties Of Tourmaline
Alluvial Tourmaline
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, 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 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, or 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 of gold. They thought that these colorful stones were emeralds and sapphires and sent them back to Portugal, where they were cut into gems and used to make jewelry for royalty and wealthy citizens. (Tourmaline was not recognized as a distinct mineral until 1793.)
Beginning in the late 1800s, a steady stream of tourmaline discoveries have been made in the pegmatite deposits of Minas Gerais, Brazil. Since then, millions of carats of tourmaline have been produced in a wide range of colors, including much bicolor material. This diverse stream of tourmaline from Brazil has been the most important source for the worldwide 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 pink and green tourmaline have been produced from dozens of Maine localities.
The most important source of tourmaline in the United States has been the tourmaline mines of southern California. Tourmaline has been mined there since the late 1800s. On the basis of cumulative dollar value, tourmaline has been the most important gem material mined in California. Most of this production occurred over 100 years ago in Riverside and San Diego Counties. Tons of red tourmaline was mined there and shipped to China, where it was used to make snuff bottles, carvings, jewelry and many other items. Today, a little tourmaline is being produced by small-scale mining. The miners today sell much of their best production as mineral specimens.
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.
Geological Properties Of Tourmaline
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 has a prismatic crystal habit and often has obvious striations that parallel the long axis of a crystal.
Tourmaline crystals often have triangular or six-sided cross-sections with rounded edges.
Tourmaline crystals are often color zoned through their cross-sections or along their length.
Tourmaline can be 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.
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.
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 usually used as an adjective. For example, “pink tourmaline” or “purple tourmaline.” “Yellow tourmaline” is sometimes sold as “canary tourmaline”.
“Color names” make 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.
Geological Properties Of 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. 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.
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 as 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 moved, and 3) when the head of the observer is moved.
Geological Properties Of Tourmaline
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.
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
Crystals By Rob Shop Home Page
Metaphysical Healing Properties Blogs
Crystal – Programming, Cleansing, Chakra Info Blogs
Top Stones List For The Holidays
Geological Properties Of Crystals
Leave a Reply
You must be logged in to post a comment.