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“Black Opal,” Harlequin Prince, 215.85 carats, Lightning Ridge, American Museum of Natural History, by greyloch is licensed under CC By-SA 2.0
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 …
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