Difference between revisions of "Tourmaline"

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(Refractive index: added info on Kerez effect and deleted that it was just for green stones)
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The indices of refraction increase with higher iron content.
 
The indices of refraction increase with higher iron content.
  
Probably due to thermal shock, some stones may show 4 (or even 8) different values per reading. This effect is sometimes seen in green tourmalines and is named the "[[Refractometer#Kerez_effect|Kerez effect]]".
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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 "[[Refractometer#Kerez_effect|Kerez effect]]". Careful recutting of the stone will reveal that it is an outer-edge phenomenom [Dietrich, 1985].
  
 
===Polariscope===
 
===Polariscope===

Revision as of 07:26, 8 October 2007

Tourmaline
Chemical composition Complex borosilicate
Crystal system Trigonal
Habit Trigonal prisms with convex faces
Fracture Conchoidal
Hardness 7.5
Optic nature Uniaxial -
Refractive index 1.62 - 1.64
Birefringence 0.014-0.021 (up to 0.039)
Dispersion Low, 0.018
Specific gravity 3.06 (3.01-3.11)
Lustre Vitreous
Pleochroism Weak to strong
Bi-color tourmaline
Photo courtesy of Lembeck Gems

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 group

Tourmaline is a large group consisting of complex borosilicates.

Species

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.

Varieties

There are many, mainly color, varieties of these species.

Color varieties (names apply to all species).

Other varieties.

  • Paraiba - neon colored elbaite tourmaline (color due to copper and manganese)

Diagnostics

Color

Tourmaline occurs in almost any color. Bi-colored specimens and "watermelons" are common.

Refractive index

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].

Polariscope

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 interference figures on lateral rotation (with a 2V up to 10°).

Magnification

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.

Phenomena

  • Cat's-eyes

Treatments

Tourmalines may be heat treated to around 700° C to lighten the color, this is a stable alteration.
Other treatments are irradiation (stable) and waxing of surface imperfections. The latter treatment is not stable.

Sources

  • Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
  • The Tourmaline Group (1985) - Richard Dietrich ISBN 0442218575