16.68: Topaz
- Page ID
- 6498
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|---|---|
| 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) |
Figure \(\PageIndex{1}\): Peach Pink Imperial Topaz
Photo courtesy of Lembeck Gems
| Topaz image gallery |
Topaz is an aluminum 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
Transparent.
Refractometer
Topaz with high concentrations of fluorine has 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
Figure \(\PageIndex{2}\): 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
- From Gems & Gemology: A New Imitation of Imperial Topaz
- Gems Their Sources, Descriptions and Identification 4th Edition (1990) - Robert Webster (6th ed.)
- Gem-A Foundation and Diploma notes
- Introduction to Optical Mineralogy 3rd edition (2003), Prof. W.D. Nesse
- Gem Reference Guide (1995) - GIA ISBN 0873110293
External links
- Son of Franken-stone: An Update on Irradiated Gemstones from Pala International