Tourmaline Mining
Description:
Tourmaline is a crystal silicate mineral compounded with elements such as magnesium , aluminum, iron, sodium, lithium. Tourmaline is classed as a semi-precious stone and the gem comes in a wide variety of colors. The name comes from the Sinhalese word "turamali" or "toramalli", which applied to different gemstones found in Sri Lanka
Physical Properties :
|
Name |
Tourmaline |
|
Varieties |
Achroite, Chrome-Tourmaline, Dravite, Elbaite, Cuprian Elbaite, Fluor-buergerite, Indicolite, Liddicoatite, Paraíba Tourmaline, Rossmanite, Rubellite, Schorl, Uvite, Verdelite, Watermelon Tourmaline |
|
Colors |
All |
|
Heat Sensitivity |
Some |
|
Fracture |
Conchoidal, Uneven |
|
Hardness |
7-7.5 |
|
Cleavage |
None |
|
Wearability |
Very Good |
|
Crystallography |
Hexagonal (trigonal). Crystals common, usually long prismatic, heavily striated along length, various terminations; also equant, acicular. |
|
Refractive Index |
Varies by species, 1.603-1.820 |
|
Birefringence |
Varies by species, 0.006–0.080, but usually around 0.020. Also note that the absorption of the O ray is sometimes strong enough to plane polarize light. At times it is totally absorbed and will show a single line in a refractometer. |
|
Dispersion |
0.017 |
|
Luminescence |
Tourmalines are usually weak to inert in UV light. Pink stones: inert to very weak red to violet. Other colors, generally inert. Stones may be chalky blue to strong blue in SW (Newry, Maine). Pink gems from Brazil may be blue or lavender in SW, and gems from Tanzania (golden yellow, brown, and green stones) are strong yellow in SW. |
|
Luminescence Present |
Yes |
|
Luminescence Type |
Fluorescent, UV-Short |
|
Absorption Spectrum |
Not diagnostic; usually weak spectra observed. Blue and Green, near total absorption to 640 nm, strong narrow band at 498 nm. Red and Pink, broad band in green, lines at 458 and 451 nm. |
|
Pleochroism |
Moderate to strong, may be two shades of body color or two separate colors. Single colors are always stronger down the C axis. See table under "Pleochroism" below. |
|
Optics |
Uniaxial (-) |
|
Optic Sign |
Uniaxial - |
|
Luster |
Vitreous |
|
Polish Luster |
Vitreous |
|
Fracture Luster |
Vitreous |
|
Specific Gravity |
2.82-3.9. See table below. |
|
Enhancements |
Heating, irradiation, acid treatment (bleaching), plastic or epoxy fillers. |
|
Typical Treatments |
Bleaching, Fracture/Cavity Filling, Heat Treatment, Irradiation |
|
Transparency |
Transparent to opaque. |
|
Phenomena |
Chatoyancy, color change, Usambara effect, iridescence, schiller (buergerite). |
|
Birthstone |
October |
|
Formula |
|
|
Etymology |
Tourmaline is from the Singhalese word turamali, meaning “mixed-colored stones.” Tourmalines were often confused with other gems. Dravite is named after the Carinthian district of Drave, Austria. Schorl is an old German mining term for unwanted minerals associated with ore. Elbaite is named after the Isle of Elba, Italy. Buergerite is named after Professor Martin J. Buerger, crystallographer and well-known research scientist. Liddicoatite is named after Richard T. Liddicoat, the former director of the Gemological Institute of America. Chromdravite is named for its composition. Uvite is named after the Uva Province, Sri Lanka. |
|
Occurrence |
Tourmaline occurs in crystalline schists; in granites and granite pegmatites (especially elbaite); in gneiss, marbles, and other contact metamorphic rocks (especially dravite, uvite). Tourmaline is also found as inclusions in quartz. |
The style of termination at the ends of crystals is asymmetrical, called hemimorphism It belongs to the trigonal crystal system and occurs as long, slender to thick prismatic and columnar crystals that are usually triangular in cross-section. Small slender prismatic crystals are common in a fine-grained granite called aplite, often forming radial daisy-like patterns. Tourmaline is distinguished by its three-sided prisms; no other common mineral has three sides. Prisms faces often have heavy vertical striations that produce a rounded triangular effect. Tourmaline is rarely perfectly euhedral. An exception was the fine dravite tourmalines of Yinnietharra, in western Australia. The deposit was discovered in the 1970s, but is now exhausted. All hemimorphic crystals are piezoelectric, and are often pyroelectric as well. Tourmaline has variety of colors. Usually, iron-rich tourmalines are black to bluish-black to deep brown, while magnesium-rich varieties are brown to yellow, and lithium-rich tourmalines are almost any color: blue, green, red, yellow, pink etc. Rarely, it is colorless. Bi-colored and multicolored crystals are common, reflecting variations of fluid chemistry during crystallisation. Crystals may be green at one end and pink at the other, or green on the outside and pink inside: this type is called watermelon tourmaline. Some forms of tourmaline are dichroic, in that they change color when viewed from different directions. Some tourmaline gems, especially pink to red colored stones, are altered by irradiation to improve their color. Irradiation is almost impossible to detect in tourmalines, and does not impact the value. Heavily-included tourmalines, such as rubellite and Brazilian paraiba, are sometimes clarity enhanced. A clarity-enhanced tourmaline is worth much less than a non-treated gem
Natural Occurrence :
- Sri Lanka
- South West Africa
- Afghanistan
- USA (California, Maine)
- Brazil
- Madagascar
- Tanzania
- Kenya
- UK Rhodesia
- Nepal
- Mozambique
- Pakistan
- Namibia and Mexico
Tourmaline has a variety of colors. Usually, iron-rich tourmalines are black to bluish-black to deep brown, and the other varieties are brown to yellow, and lithium-rich tourmalines are almost any color: red, green, blue, yellow, pink etc.
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