Gemologists can determine the color, clarity, and cut grades of gems like diamonds and rubies reasonably well and usually with a consensus. Disagreements on borderline cases are few. Appraising opal, on the other hand, presents an entirely different situation. It’s more like an art. Each stone is an individual and can have widely varying properties. Values for grades, like G and SI1 for diamonds, can’t be simply looked up and applied. The individual grading factors need to be determined and then carefully weighed.
That said, there is a unique terminology for opals that simplifies grading and pricing as well as a set of standards for appraising opals.
Opal Terminology
In 1997, the Gemmological Association of Australia (GAA) recommended a new nomenclature for opals which has been accepted by major gem labs around the world.
Body Tone
Central to the GAA nomenclature is the body tone description. Body tone is the base color of the stone, without taking the play of color into account. All tones from black to white are assigned a value from N1 to N9.
Under this system, any opal with a body tone of N4 or darker can…
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