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COLOURED STONE GRADING NOMENCLATURE

 Colour is described by three components--hue, tone, and saturation. Hue is the dominant colour and any additional colours visible in a stone. Examples are orangy red or bluish green. Tone is the lightness or darkness of the colour from colourless to black. Saturation is the strength of the colour from grayish or brownish to vivid.

Clarity for coloured stones will be graded by the same nomenclature as diamonds (FL, VVS, VS, etc.). However, some coloured stones naturally grow with more inclusions. Therefore, the system is amended depending on the type of gemstone being graded. A TYPE I gemstone grows flawless or nearly so and would be graded strictly like a diamond. A TYPE II gemstone grows somewhat included and inclusions are more common. Grading is more lenient. A TYPE III gemstone is almost always included and grading is most lenient.

Cut for coloured stones will examine the proportions and finish looking at factors such as outline balance, depth, bulge, windowing, extinction, polish, and symmetry.

When the above factors are analyzed, the final grading will be assessed according to the standards as set forth in The Guide . Grading used will be Commercial (Lower, Middle, or Upper), Good, Fine, or Extra Fine.


COLOURED STONE TREATMENTS

Coloured stones are often subjected to various treatments to improve their appearance. Treatments are considered usual and customary practices when properly disclosed and when done without intent to defraud the consumer. Many treatments are stable and do not require special care. Others may require special care. When a treatment is detected and considered not to be usual and customary it will be so noted by this appraiser. Some treatments are reversible and re-treatable. It may be beyond the scope of an appraisal to determine exact treatment methods or the amount of treatment present. Some treatments require sophisticated equipment to identify, not found in a standard gemological laboratory.

All treatments should be disclosed to the purchaser. Market values are generally based on these treatments. Unless otherwise stated, the coloured stones in this appraisal are assumed to have been subjected to one or more of these treatments, when it is common for that species or variety, and valued accordingly. Treatments, when detected will be noted on the appraisal. Untreated gemstones, when detected and when the value is affected, will be noted. Unstable treatments or other treatments that adversely affect the value will also be noted in the appraisal when detected.

The following list describes some common gemstone treatments. It is not inclusive of all treatments.

 
Gemstone
Care of Gem
Treatment
Frequency
Stability
  Ruby Heat, possible residue Usually Excellent Normal
  Sapphire Heat Usually Excellent Normal
  Emerald Oil Opticon or other polymer resins Usually Fair to good Caution
  Tanzanite Heat Always Excellent Caution
  Pearl Bleach Usually Caution Caution
  Opal Oil, wax, epoxy Rare Fair Caution
  Blue Topaz Irradiated Always Excellent Normal

Other gemstone treatments include irradiation, dye, various coatings, and diffusion.
Some treatments such as dye may be considered fraudulent if not properly disclosed.

 
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