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16.68: Topaz

  • Page ID
    6498
  •  

    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 aquamarinezirconcitrineperidotberyl 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 apatiteandalusitedanburite 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.

     

    External links