carat

Carat, abbreviated ct, is a unit of mass used to express the weight of gemstones, especially diamonds. One carat equals 0.2 grams or 200 milligrams. The carat is subdivisible into 100 points, so a 0.25 ct stone weighs 25 points. Carat weight is commonly reported with two decimal places (for example, 1.23 ct).

History and standardization: The term carat derives from carob seeds historically used as balance weights to compare gem weights, though the seeds vary in weight. In modern practice, the carat is defined exactly as 0.2 grams, a standard codified by international metrology and trade bodies. The abbreviation ct is widely used in gemology and jewelry retail to indicate weight.

Relation to other uses: The term karat or karat is used for gold fineness, not weight, with 24-karat gold indicating pure gold. Carat weight affects the value of a gemstone, but other factors such as cut, color, and clarity also influence price. For small stones, “points” describe fractions of a carat (for example, 75 points equals 0.75 ct). Carat weight is a key specification in grading, appraisal, and sale of gemstones.