16.68: 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)

File:Imperial4.jpg

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: n_α = 1.606-1.634, n_β =1.609-1.637 , n_γ = 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