A metric carat is the the standard unit of weight used for diamonds and other gemstones. This internationally adopted weight standard equals 200 milligrams, or a fifth of a gram. The commonly used abbreviation used for carat is “ct.” The word carat is derived from the Ancient Greek word “keraton,” or carob tree. It was discovered the tree’s seeds were remarkably similar and stable in weight, and they became accepted as units for measurement.

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