Difference between revisions of "Topaz"

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(Sources)
(Sources)
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* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
 
* ''Gems, Their Sources, Descriptions and Identification'' 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
 
* ''Gem-A'' Foundation and Diploma notes
 
* ''Gem-A'' Foundation and Diploma notes
 +
* ''Introduction to Optical Mineralogy'' (2004) - William D. Nesse ISBN 0195149106

Revision as of 07:43, 2 July 2007

Topaz
Chemical composition Al2(F,OH)2SiO4
Crystal system Orthorhombic
Habit Prismatic with pyramidal/domed terminations
Cleavage Perfect, basal plane
Fracture Sub-conchoidal to uneven
Hardness 8
Optic nature Biaxial +
Refractive index 1.606-1.644
Birefringence 0.008 - 0.010
Dispersion Low, 0.014
Specific gravity 3.49 - 3.57
Lustre Vitreous
Pleochroism Distinct to strong (depending on body color)
Peach Pink Imperial Topaz
Photo courtesy of Lembeck Gems

Topaz is an aluminium silicate with varying amounts of fluorine (F) and hydroxyl (OH) which replace eachother through isomorphous replacement. Extreme outer limits with only fluorine or only hydroxyl have not been reported.


Diagnostics

In color and diaphaneity, topaz can be confused with many stones like aquamarine, zircon, citrine, peridot and sapphire.

Color

Topaz is allochromatic and occurs in many colors.

  • colorless
  • yellow
  • green
  • blue
  • red
  • pink
  • brown

Diaphaneity

Transparant

Refractometer

Topaz with high concentrations of fluorine have a lower refractive index (1.61-1.62)than those with high concentrations of hydroxyl (1.63-1.64).

Full refractive index range: nα = 1.606-1.634, nβ =1.609-1.637 , nγ = 1.616-1.644 with a maximum birefringence of 0.008-0.010 (depending on content of fluorine and hydroxyl).

Other stones falling in the refractive index range are apatite, andalusite, danburite and tourmaline.

Specific gravity

As with the refractive index, the specific gravity changes with high concentrations of hydroxyl and fluorine.
Hydroxyl causes a lower specific gravity (3.53) while fluorine raises the specific gravity of topaz (3.56).

Topaz sinks in all common heavy liquids while apatite, andalusite, danburite and tourmaline will float in methylene iodide (sg = 3.33).

Dichroscope

The pleochroism is usually moderate and almost dichroic, except for heated pink stones where it is more profound.

Treatments

Irradiation to create blue stones and heat treatment of brownish stones to create pink stones.

Imitations

Recently,2007, yellow-orange-pink flame-fusion corundum is offered in Minas Gerais, Brazil as imperial topaz.

Synthetics

Although topaz is synthesized, the material is not commercially available.

Sources

  • From Gems & Gemology: A New Imitation of Imperial Topaz
  • Gems, Their Sources, Descriptions and Identification 4th ed. (1990) - Robert Webster ISBN 0750658568 (6th ed.)
  • Gem-A Foundation and Diploma notes
  • Introduction to Optical Mineralogy (2004) - William D. Nesse ISBN 0195149106