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Smithsonian National Museum of Natural History

Carats, French Carats, and Grains

A carat is a unit of mass, originally based on the weight of a carob seed, which naturally vary in size and weight. In 1913, in an attempt to standardize the carat, the United States adopted the “metric carat,” equivalent to 200 mg or 0.200 g. The weight of diamonds, and all other gemstones, is now based on the metric carat. The carat weights familiar from jewelry stores are, in fact, metric carats.

Older sources often give gem weights in archaic units. For example, the weight of Tavernier's Diamond was 112 316 old French carats (a unit similar to but slightly smaller than a metric carat), and the weight of the blue diamond described in the Francillon Memo was 177 grains (one grain is approximately one-third of a metric carat).

The accepted metric carat weights of the Hope Diamond and its precusor stones are as follows:

Tavernier's Diamond~115 carats
French Blue Diamond~69 carats
Hope Diamond45.52 carats

Creator Stephen Attaway
Source Attaway Gems

Computer models depicting the history of the Hope Diamond, including (counterclockwise from top) the Tavernier, the French Blue, and the Hope Diamond.

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