Tourmaline
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 |
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 east. 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.
Contents
Tourmaline group
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
- Indicolite - blue tourmaline
- Verdelite - green tourmaline
- Siberite - reddish-violet tourmaline
- Watermelon - a pink core with green edges
- Bi-colored - two colored tourmaline
- Tri-colored - three colored tourmaline
Other varieties.
- Paraiba - neon colored tourmaline with high copper content (elbaite)
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, some stones may show 4 (or even 6) different values per reading. This effect is sometimes seen in green tourmalines and is named the "Kerez effect".
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.