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The name opal is believed to be derived from the Sanskrit word upala, which means 'jewel', and later the Greek derivative opállios, which means 'to see a change in color'.
There are two broad classes of opal: precious and common. Precious opal displays play-of-color; common opal does not.
Black opal is considered the rarest, while white, gray, and green opals are the most common.