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Tourmaline


Properties
Chemical Composition: 
A complex silicate - WX3Y6B3Si6O27 (O,OH,F)4. In elbaite tourmaline X = mainly Li & Al, W = Na, Y = Al. Mg, Fe, Ca, etc., replace isomorphously.
Classification / Type: 
The tourmaline group is made up of a number of different members. Of gem interest are the Elbaite tourmalines which are found in many colors; other related varieties such as dravite tourmaline (brown), schorl tourmaline (black), indicolite tourmaline (blue), rubellite tourmaline (red), liddicoatite tourmaline etc. are also being used as gemstones. Since the distinctions are of a purely chemical nature, all these are considered as a single species - tourmaline.
Colors / Varieties: 
All colors. Transparency: Transparent to Opaque. Varieties: Parti-colored and chatoyant.
Crystal System / Forms: 
Trigonal System / Prismatic habit showing hemimorphism; deep vertical striations / grooves on the prism face are characteristic.
Hardness: 
7 - 7.5
Specific Gravity: 
  • Pink tourmaline: 3.03
  • Red and pale green tourmaline: 3.05
  • Brown tourmaline: 3.06
  • Dark Green tourmaline: 3.08
  • Blue, yellow tourmaline: 3.10
  • Black tourmaline: more than 3.15
Cleavage / Fracture: 
None / Conchoidal
Optic Character: 
Anisotropic, D.R.; Uniaxial negative.
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.624 - 1.644 / 0.020. Range: 1.618 - 1.650
Pleochroism: 
Distinct dichroism with strong absorption along c-axis. Varies with color.
Dispersion: 
0.017
Magnification: 
Liquid inclusions, trichites (fine liquid capillaries or tubes), highly reflecting inclusions, phase and crystal inclusions.
U.V. Fluorescence: 
Weak to inert in longwave and varies with colors in shortwave.
Spectrum: 
Varies as per composition.
Cause of Color: 
  • Red and pink tourmaline: Manganese (some pink - titanium)
  • Blue tourmaline: Iron
  • Green tourmaline: Iron / Chromium / Vanadium
  • Purple tourmaline: Iron and Manganese
Treatment (Enhancement): 
Specific Tests & Remarks: 
  • Pyroelectric: Develops an electrical charge when heated.
  • Piezoelectric: Develops and electrical charge when pressure is applied parallel to the c-axis direction. This is made use of in depth recording apparatus in deep sea explorations.
Synthesis: 
-
Simulants (with key separation tests): 
Aquamarine (R.I., S.G., inclusions), Andalusite (optic figure, pleochroism, birefringence), Apatite (hardness, birefringence, spectrum), Danburite (optic figure, R.I., birefringence, pleochroism), Iolite (pleochroism, optic figure, R.I., S.G.), Beryl (R.I., birefringence, S.G., pleochroism), Citrine (R.I., S.G., pleochroism), Tsavorite Garnet (optic character), Chrome Diopside (optic figure, R.I., birefringence, S.G.), Red Spinel (optic character), etc.
Geological Occurrence: 
In crystalline schists, in granites, pegmatite.
Sources: 
Brazil (from Paraiba known as Paraiba tourmaline), South-west Africa, Russia (Ural Mountains), Zambia, Madagascar, Tanzania, Sri Lanka, Kenya, Pakistan, Myanmar, etc.
Cuts & Uses: 
Faceted cuts, cabochon, beads in jewelry, carvings.
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