Step 4: Gem Grading
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.
In 1997, the Gemmological Association …