|Habit||Prismatic with pyramidal/domed terminations|
|Cleavage||Perfect, basal plane|
|Fracture||Sub-conchoidal to uneven|
|Optic nature||Biaxial +|
|Birefringence||0.008 - 0.010|
|Specific gravity||3.49 - 3.57|
|Pleochroism||Distinct to strong (depending on body color)|
Topaz is an aluminium silicate mineral 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.
As a gemstone, topaz is the birthstone of November and imperial topaz is used to celebrate the 23rd year of marriage.
In color and diaphaneity, topaz can be confused with many gemstones like aquamarine, zircon, citrine, peridot, beryl and sapphire.
All of these, with the exeption of peridot are uniaxial while topaz is biaxial. Peridot is also biaxial, but has higher refractive indices.
Topaz is allochromatic and occurs in many colors.
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 charcter of topaz is biaxial with a possitive 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).
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).
The pleochroism is usually moderate and almost dichroic, except for heated pink stones where it is more profound.
Topaz has perfect cleavage in the direction of the basal plane (001), so care should be taken not to knock the gemstone.
Irradiation to create blue stones and heat treatment of brownish stones to create pink gemstones.
Recently,2007, yellow-orange-pink flame-fusion corundum is offered in Minas Gerais, Brazil as imperial topaz.
Although topaz is synthesized, the material is not commercially available for gemstones.
- 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