In reference to gemstones, an abbreviation of carat, a unit of weight. One carat is equal to 0.20 grams.
In reference to gold, also an abbreviation of carat, a unit of purity or fineness of gold and gold alloy, expressed as a number out of 24 parts by weight, e.g. '24 carat' signifies pure gold, '18 carat' 18/24th gold in the alloy, et cetera.
Originally derived from the carob seed, called quirat in Arabic, a seed of naturally uniform weight.