Tourmaline

tourmaline
Tourmaline cabochons in various colors

Tourmaline is a crystal boron silicate mineral compounded with elements such as aluminium, iron, magnesium, sodium, lithium, or potassium. Tourmaline is classified as a semi-precious stone and the gem comes in a wide variety of colors. The name comes from the Sinhalese word “Thuramali” or “Thoramalli” which applied to different gemstones found in Sri Lanka.

Brightly colored Sri Lankan gem tourmalines were brought to Europe in great quantities by the Dutch East India Company to satisfy a demand for curiosities and gems. At the time it was not realised that schorl and tourmaline were the same mineral.

Source: en.wikipedia.org/wiki/Tourmaline

Tourmaline Pictures

Group of tourmaline exhibiting vertical striationsPink tourmaline roughRough tourmaline

Cause of Color :
  • Red and pink tourmaline: Manganese (some pink – titanium)
  • Blue tourmaline: Iron
  • Green tourmaline: Iron / Chromium / Vanadium
  • Purple tourmaline: Iron and Manganese
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.

Crystal System / Forms :

Trigonal System / Prismatic habit showing hemimorphism; deep vertical striations / grooves on the prism face are characteristic.

Cuts & Uses :

Faceted cuts, cabochon, beads in jewelry, carvings.

Dispersion : 0.017
Hardness : 7 - 7.5
Lustre : Vitreous
Magnification :

Liquid inclusions, trichites (fine liquid capillaries or tubes), highly reflecting inclusions, phase and crystal inclusions.

Optic Character :

Anisotropic, D.R.; Uniaxial negative.

Pleochroism : Distinct dichroism with strong absorption along c-axis. Varies with color.
Refractive Index / Birefringence :

1.624 – 1.644 / 0.020. Range: 1.618 – 1.650

Simulants (with 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.

Sources :

Brazil (from Paraiba known as Paraiba tourmaline), South-west Africa, Russia (Ural Mountains), Zambia, Madagascar, Tanzania, Sri Lanka, Kenya, Pakistan, Myanmar, etc.

Specific Tests :
  • 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.
Spectrum :

Varies as per composition.

Transparency :

Transparent to Opaque.

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