The carat (abbreviated ct) is the internationally standardised unit of mass used to weigh gemstones and pearls. One metric carat equals exactly 200 milligrams, or one-fifth of a gram. It is subdivided into 100 points, so that a stone weighing 0.75 ct may equally be described as 75 points. The metric carat was formally adopted by the Fourth General Conference on Weights and Measures in 1907 and has since been enshrined in the national standards of virtually every gem-trading nation, making it the rare example of a unit that is genuinely universal across the jewellery trade.

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Etymology and historical background

The word derives, via Italian and Arabic, from the Greek keration, meaning the seed of the carob tree (Ceratonia siliqua). Carob seeds were long believed to be remarkably consistent in weight and were used as counterweights on balance scales by traders across the Mediterranean and the Arab world. In practice, individual carob seeds vary considerably, and the consistency of the unit before standardisation depended more on local convention than on botanical fact. Regional carat values in use before 1907 ranged from approximately 188 mg to 213 mg, which introduced meaningful discrepancies in cross-border transactions — a problem the metric carat was designed to eliminate.

The metric carat in practice

Carat weight is recorded to two decimal places in virtually all laboratory reports and commercial invoices (e.g., 2.34 ct), with a third decimal place sometimes noted but rounded for the official weight. In the diamond trade specifically, the point system remains in everyday use: a 0.50 ct stone is a "fifty-pointer," and a 1.00 ct stone is described as "one carat" or "a hundred points." This shorthand is less commonly applied to coloured gemstones, where weight is typically stated in decimal carats alone.

Carat weight is one of the Four Cs of diamond grading — alongside cut, colour, and clarity — as codified by the Gemological Institute of America (GIA). Across all gem species, weight is a primary determinant of per-carat price, and the relationship is non-linear: a fine 5 ct ruby does not simply cost five times as much as a comparable 1 ct ruby, but many times more, because large, high-quality stones are exponentially rarer. Significant price steps, known in the trade as magic sizes, occur just below round-number thresholds (0.50 ct, 1.00 ct, 2.00 ct, and so on), where demand is concentrated.

Carat versus karat

The terms carat and karat are etymologically identical but refer to entirely different quantities. In British usage, "carat" serves both functions — gemstone mass and gold purity — distinguished by context: a "1 carat diamond set in 18 carat gold" is unambiguous in practice. American usage reserves "karat" (abbreviated kt or K) exclusively for gold purity, while retaining "carat" (ct) for gemstone weight. Gold purity expressed in carats is a ratio out of 24 parts: 18 carat gold contains 18 parts gold per 24, or 75 per cent pure gold. The two systems share no mathematical relationship and should never be conflated.

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Weight estimation and density

When a stone is mounted and cannot be removed for direct weighing, gemmologists estimate carat weight using formulae based on measured dimensions and the known specific gravity of the material. These formulae differ by species and cut style — a round brilliant diamond, a cushion-cut sapphire, and a cabochon emerald each require a different calculation — because density varies significantly across gem minerals. Diamond has a specific gravity of approximately 3.52, while corundum (ruby and sapphire) is approximately 4.00, and demantoid garnet approximately 3.84. A ruby and a diamond of identical physical dimensions will therefore differ noticeably in carat weight.

Laboratory reporting

All major independent gemmological laboratories — including GIA, Gübelin, SSEF, and Lotus Gemology — state carat weight on their reports as a directly measured value, typically obtained on a calibrated electronic balance accurate to 0.001 ct or better. Weight is considered an objective, instrument-derived datum and is not subject to the interpretive variation that affects colour or clarity grades. For this reason, carat weight is the single element of a gem's description least likely to differ between laboratories or between buyer and seller.