Topaz

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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 image gallery

Topaz is an aluminium silicate mineral with varying amounts of fluorine (F) and hydroxyl (OH) which replace each other through isomorphous replacement. Extreme outer limits with only fluorine or only hydroxyl have not been reported.
As a gemstone, topaz is the birthstone of November and imperial topaz is used to celebrate the 23rd year of marriage.


Diagnostics

In color and diaphaneity, topaz can be confused with many gemstones like aquamarine, zircon, citrine, peridot, beryl and sapphire.
All of these, with the exception of peridot are uniaxial while topaz is biaxial. Peridot is also biaxial, but has higher refractive indices.

Color

Topaz is allochromatic and occurs in many colors.

  • Colorless
  • Yellow - colored by color centers
  • Green
  • Blue - colored by color centers, irradiation/heat treatment
  • Red - colored by chromium
  • Pink - colored by chromium, heat treatment
  • Orange - colored by color centers and chromium
  • Brown - colored by color centers

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).

The optic character of topaz is biaxial with a positive optic sign
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.

Durability

Topaz has perfect cleavage in the direction of the basal plane (001), so care should be taken not to knock the gemstone.

Phenomena

Cat's-eyes.

Inclusion images

2-phase inclusions in colorless topaz.
Photo courtesy of John Huff, gemcollections.com


Topaz inclusions gallery

Treatments

Irradiation followed by heat treatment to create blue stones and heat treatment of brownish stones to create pink gemstones.
The irradiation process to create blue stones could make them radioactive and a "cooling down" period is usually taken into account. After that period these stones are perfectly safe.

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 for gemstones.

Sources

External links