The term "carat" goes back to the traders of the ancient world. A standard weight was required for precious gems as merchants of the ancient Mediterranean and Middle East were dependent on the ability to trade with a reasonably consistent unit of measurement. It was this need that lead to the adoption of seeds and grains as widespread accepted units of measurement.

The carob seed and the wheat grain, both of which had been used for food purposes, were found to be ideal units of weight. This was a result of the fact that any given carob seed weighs, for all practical purposes, the same as any other given carob seed. In addition, any given grain of wheat has an identical weight to any other grain of wheat. In time, the carob seed became the standard unit of weight for gems. The word carob itself comes from the Arabic word for the carob seed, the quirat. Through the rise and fall of civilizations, this word became corrupted through to the Greek, keration, which eventually became what we know today as the carat.

For centuries the carob seed remained the weight measurement for precious gems. By the middle ages, however, changes in trade routes had occurred, and large centers of trade were now found within Europe. The carat, as it had become known, became linked to 4 grains Troy weight, with the carob seed having been abandoned at some point during the shift of trade centers. The Troy carat was the equivalent of approximately 205 milligrams. This measurement of weight lasted for the carat until the 20th century. It was between 1907-1914 that the carat was married to the metric system of weights. By 1914 the United States officially abandoned the former Troy measurement of 205.3 milligrams for the carat, and adopted the current metric carat measurement of 200 milligrams.