|Chemical composition||Complex borosilicate|
|Habit||Trigonal prisms with convex faces|
|Optic nature||Uniaxial -|
|Refractive index||1.62 - 1.64|
|Birefringence||0.014-0.021 (up to 0.039)|
|Specific gravity||3.06 (3.01-3.11)|
|Pleochroism||Weak to strong|
Tourmaline is an extremely complex borosilicate that occurs in more than 100 colors. It is hard and durable and very well suited for jewelry. It is a pyroelectric mineral, meaning that when warmed, it attracts dust and other lightweight particles. The Dutch later noticed this property and called the crystals "aschentreckers," and used them to pull ashes out of tobacco pipes. It wasn't introduced into Europe until the early 1700's, when it was imported from the Ceylon by the Dutch. Shortly thereafter it was declared a stone of the Muses, inspiring and enriching the creative processes. It was a talisman for artists, actors and writers. Today, it is mined extensively in South America, East Africa, and in San Diego County, California.
Tourmaline is a large group consisting of complex borosilicates.
Only 5 species of tourmaline are of real importance to gemologists.
From the above 5, elbaite is the most important one. Discrimination between elbaite and liddicoatite is usually not attempted.
There are many, mainly color, varieties of these species.
Color varieties (names apply to all species).
- Achroite - colorless tourmaline
- Rubellite - red tourmaline (color due to iron and manganese)
- Indicolite - blue tourmaline (color due to iron)
- Verdelite - green tourmaline (color due to iron and titanium)
- Siberite - reddish-violet tourmaline
- Watermelon - a pink core with green edges
- Bi-color - two colored tourmaline
- Tri-Color - three colored tourmaline
- Paraiba - neon colored elbaite tourmaline (color due to copper and manganese)
Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.
The refractive index of tourmaline lies between 1.610 and 1.698 (usually between 1.62 and 1.64) with a birefringence up to 0.039 (usually 0.019).
nω = 1.631-1.698, nε = 1.610-1.675, optic sign is negative.
The indices of refraction increase with higher iron content.
Probably due to thermal shock (and/or heat treatment), some stones may show 4 (or even 8) different values per reading. This effect is named the "Kerez effect". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].
Some dark colored tourmalines have a so called "closed axis" due to strong selective absorption in the direction of the optic axis and an interference figure may be hard (if not impossible) to find in that case.
Lighter colored stones may be cut with the optic axis perpendicular to the table and good interference figures can be found there.
Some tourmalines show pseudo-biaxial (due to internal stress) interference figures on lateral rotation with a 2V up to 25° [Nesse, 2004; Dietrich, 1985].
Tourmalines can be of type I to type III clarity grades.
Typical inclusions are:
- Trichites (small thread-like twists)
- Flattened liquid channels running parallel to the optic axis.
- Liquid Veils
- 2 and 3-Phase Inclusions
- Hollow tubes
Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.
A process which seems to work well for deep saturated reds from Nigeria involves slowly ramping the furnace at a rate of 125° C per hour to 520° C, holding the latter temperature for 2 hours and then letting the furnace cool completely.
A deep red Nigerian stone in the gallery below has been heat treated several times as an experiment by Roger Dery, with the ramping done last.
Other treatments are irradiation - for example with cobalt-60 - (stable) and waxing of surface imperfections. The latter treatment is not stable.
Cobalt-60 irradiation gives to rise pink and hot pink colors in tourmaline.
- Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
- The Tourmaline Group (1985) - Richard Dietrich ISBN 0442218575
- Introduction to Optical mineralogy (2004) - William D. Nesse ISBN 0195149106
- Refraction Anomalies in Tourmalines - R. Keith Mitchell, Journal of Gemmology Vol. 10, 194 (1967)