The metric carat is the internationally standardised unit of mass used to weigh gemstones and pearls. Defined as exactly 0.200 grams (200 milligrams), it is abbreviated ct and subdivided decimally, so that a stone of one and a half carats is recorded as 1.50 ct. In the diamond trade, the carat is further divided into 100 points, making a 0.75 ct stone a "75-pointer" in everyday dealer parlance. The metric carat was adopted by international agreement in 1907, ending centuries of regional variation, and today it is the sole legal unit of gem weight in virtually every jurisdiction that regulates the jewellery trade.

Historical Background

The word "carat" derives, via Arabic qīrāṭ and Greek kerátion, from the carob tree (Ceratonia siliqua), whose seeds were long believed to be remarkably uniform in mass and were therefore used as counterweights on balance scales throughout the Mediterranean and Middle East. In practice the carob seed is not as consistent as tradition suggests — individual seeds vary by as much as 15 per cent — but the belief in their uniformity was sufficient to anchor a widespread convention.

Before standardisation, the carat varied considerably from city to city and country to country. The Florence carat differed from the Amsterdam carat; the London carat differed from both. Values ranged roughly between 188 mg and 213 mg, meaning that a stone described as "one carat" could represent meaningfully different masses depending on where it was sold. The confusion was commercially damaging and scientifically incoherent. In 1907, the Fourth General Conference on Weights and Measures, together with the International Committee of Weights and Measures, formalised the metric carat at exactly 200 mg, aligning it with the metric system. Most major trading nations had adopted this standard by the 1930s, and it is now embedded in the legislation of the United States, the European Union, the United Kingdom, and all significant gem-trading nations.

Notation and Subdivision

Carat weights are always expressed to two decimal places in formal gemmological and trade contexts — 0.50 ct, 1.23 ct, 5.08 ct — because laboratory balances used in gem grading are calibrated to the nearest 0.01 ct (2 mg). The Gemological Institute of America (GIA) and other major grading laboratories record weight to the nearest hundredth of a carat on certificates, rounding down when the third decimal place is 0–4 and up when it is 5–9, though some laboratories, notably for very small stones, report to the nearest 0.001 ct.

In the diamond trade specifically, the point system is common in verbal and written shorthand:

• 1 ct = 100 points
• 0.50 ct = 50 points (a "half-carat" or "50-pointer")
• 0.25 ct = 25 points (a "quarter-carat")
• 0.10 ct = 10 points (a "ten-pointer")

The point system is rarely applied to coloured gemstones, where decimal notation is universal.

Carat Weight as a Value Determinant

Carat weight is one of the Four Cs of diamond grading — alongside cut, colour, and clarity — as codified by the GIA in the mid-twentieth century. For coloured gemstones, weight is equally fundamental but interacts differently with value: because fine ruby, emerald, and sapphire are denser or less dense than diamond, a stone of identical carat weight will appear physically larger or smaller depending on species. A 1.00 ct round brilliant diamond measures approximately 6.5 mm in diameter; a 1.00 ct round ruby, owing to corundum's higher specific gravity (approximately 4.00 g/cm³ versus diamond's 3.52 g/cm³), measures closer to 6.0 mm. Conversely, a 1.00 ct round emerald, with a specific gravity of roughly 2.72 g/cm³, measures approximately 6.7–6.8 mm.

Across virtually all gem species, price per carat rises non-linearly with weight. A fine 5.00 ct Burmese ruby does not cost five times as much as a comparable 1.00 ct stone; it may command ten to twenty times the per-carat price, because large, high-quality specimens are exponentially rarer. This non-linearity is most pronounced in ruby, emerald, alexandrite, and Kashmir sapphire, where stones above 3 ct of top quality are genuinely scarce on the world market. Certain weight thresholds — 1.00 ct, 2.00 ct, 3.00 ct, 5.00 ct, 10.00 ct — carry psychological and commercial significance, and stones just below these benchmarks (0.98 ct, 1.97 ct) are sometimes described as "shy" weights in the trade.

Measurement in Practice

Loose gemstones are weighed on electronic balances accurate to 0.001 g (1 mg), calibrated with certified reference weights traceable to national metrology standards. For mounted stones, where direct weighing is impossible, gemmologists use empirically derived formulae based on measured dimensions. These formulae differ by species (to account for differing specific gravities) and by cut shape. For a standard round brilliant diamond, the most widely used approximation is:

Estimated weight (ct) = diameter² × depth × 0.0061

where diameter and depth are in millimetres. Equivalent formulae exist for ovals, cushions, emerald cuts, and other standard shapes. Such estimates carry an uncertainty of roughly ±5–10 per cent and are used for insurance valuation and estate appraisal rather than for grading certificates, which always report directly measured weight.

Distinction from Karat

The carat (ct) used for gemstone weight must not be confused with the karat (k or kt) used to express the purity of gold alloys. The two terms share an etymological root — both descend from the same carob-seed tradition, since the purity of gold was historically assessed by weighing a sample against carob seeds — but they measure entirely different properties. A karat of gold purity represents one twenty-fourth part of pure gold by mass (18k gold is 18/24 = 75% pure gold), and has no dimensional or mass equivalent to the gemstone carat. In British English, "carat" is used for both concepts, distinguished by context ("a 1.50 carat sapphire set in 18-carat gold"); in American English, "karat" is reserved exclusively for gold purity, which reduces ambiguity.

Regulatory and Laboratory Context

Major gemmological laboratories — including the GIA, Gübelin Gem Lab, SSEF Swiss Gemmological Institute, and Lotus Gemology — report carat weight on all grading reports for loose stones. The weight is typically the first datum listed, as it is the most objective and reproducible measurement in the grading process. Laboratory balances are re-calibrated at regular intervals, and the reported weight is considered definitive for trade and legal purposes. In many jurisdictions, misrepresenting the carat weight of a gemstone in a commercial transaction constitutes fraud under consumer-protection legislation.

Further Reading

• GIA — The 4 Cs: Carat Weight
• Gems & Gemology (GIA peer-reviewed journal)
• AGTA — Gemstone Education