Few terminological distinctions in the jewellery trade carry more practical consequence than the difference between carat and karat. Though pronounced identically in English and sharing a common etymological root, the two terms denote entirely separate systems of measurement: carat is the internationally standardised unit of mass for gemstones, while karat (in American and British usage) or carat (in much of continental Europe and elsewhere) expresses the proportional purity of gold in an alloy. Confusing the two in a laboratory report, a hallmark, or a sales document is not merely an embarrassment — it can constitute a regulatory violation under the labelling rules of bodies such as the United States Federal Trade Commission (FTC) and the international jewellery confederation CIBJO.

The Metric Carat: Gemstone Weight

The carat as a unit of gemstone weight has ancient origins, almost certainly derived from the carob seed (Ceratonia siliqua), whose dried seeds were historically used as counterweights on balance scales in Mediterranean and Middle Eastern markets. For centuries the precise weight of a "carat" varied from city to city — a Venetian carat differed from a Florentine one, and both differed from the carat used in Bombay or Amsterdam. This inconsistency created persistent difficulties in international gem trading.

Standardisation came in 1907, when the Fourth General Conference on Weights and Measures formally adopted the metric carat, defined as exactly 0.200 grams (200 milligrams). The United States adopted the metric carat in 1913; most trading nations followed within a generation. Today the metric carat is universally employed in gemmological laboratories, auction catalogues, and trade invoices worldwide. It is abbreviated ct for a single stone and cts or ct TW (total weight) for multiple stones set together.

Fractions of a carat are expressed in points: one point equals one-hundredth of a carat (0.01 ct, or 2 milligrams). A stone described as "fifty points" weighs 0.50 ct. GIA laboratory reports and those of other major grading laboratories — Gübelin, SSEF, Lotus Gemology, AGL — always state gemstone weight in metric carats to two decimal places, occasionally to three for very small or very large stones where precision is commercially significant.

The Karat: Gold Purity

The karat system for expressing gold purity operates on an entirely different logic. Pure gold is defined as 24 karats (abbreviated kt or K in North American usage, and ct in British and many Commonwealth contexts). An alloy described as 18 karat contains 18 parts gold to 6 parts other metals — precisely 75.0 per cent gold by mass. The arithmetic is straightforward: divide the karat number by 24 to obtain the gold fraction.

Common commercial alloys and their gold content are as follows:

• 24 kt — 999 fine (99.9% gold); too soft for most jewellery applications.
• 22 kt — 916 fine (91.6% gold); traditional in South Asian and Middle Eastern jewellery.
• 18 kt — 750 fine (75.0% gold); the dominant standard in European fine jewellery and Swiss watchmaking.
• 14 kt — 585 fine (58.5% gold); widely used in North American commercial jewellery.
• 10 kt — 417 fine (41.7% gold); the legal minimum to be sold as gold in the United States.
• 9 kt — 375 fine (37.5% gold); the legal minimum in the United Kingdom and Ireland.

The millesimal fineness system — expressing purity as parts per thousand (e.g., 750, 585, 375) — is the hallmarking standard used across the European Union and in most national assay offices. It is numerically equivalent to the karat fractions above and appears stamped directly on jewellery alongside the assay office mark and maker's mark.

The historical origin of the karat for gold parallels that of the gemstone carat: both trace to the carob seed counterweight tradition, and both entered European commercial practice through medieval Mediterranean trade. The divergence in spelling — "karat" for gold, "carat" for gems — is largely an Anglo-American convention that hardened during the twentieth century as regulatory frameworks demanded unambiguous labelling.

Spelling Conventions by Region

The orthographic distinction between the two terms is not universal, and practitioners working across jurisdictions must be alert to local convention.

• United States: "Karat" (abbreviated K or kt) is mandated by the FTC for gold purity; "carat" (abbreviated ct) is used for gemstone weight. The FTC's Guides for the Jewelry, Precious Metals, and Pewter Industries explicitly govern these usages.
• United Kingdom: "Carat" is used for both gem weight and gold purity, with context and abbreviation (ct for gems; ct or the fineness stamp for gold) providing disambiguation. The UK Hallmarking Act 1973 and the British Hallmarking Council oversee gold marking.
• Continental Europe, Australia, and most of Asia: "Carat" is similarly used for both, with millesimal fineness stamps on metal items removing ambiguity in practice.
• CIBJO (The World Jewellery Confederation): The CIBJO Blue Books — the authoritative international standards documents for diamonds, coloured stones, pearls, and precious metals — acknowledge both conventions but require that labelling be unambiguous in context.

For international gemmological and auction purposes, the safest practice is to write "metric carat" or "ct" for gemstone weight and to express gold purity as a millesimal fineness (e.g., 750) or to spell out "18-karat gold" — a formulation that is unambiguous in any English-speaking market.

Regulatory and Trade Implications

The distinction matters beyond mere pedantry. A jewellery retailer in the United States who marks a gold item with "ct" rather than "kt" risks a labelling violation under FTC guidelines. Conversely, a laboratory report that expresses a diamond's weight in "karats" would be considered erroneous by any accredited grading body. In auction catalogues, misuse of the terms — particularly the description of a gold alloy as "18 carat" in a North American context — can create confusion about whether a weight or a purity is being stated, with potential legal consequences in disputes over material misrepresentation.

CIBJO's precious metals standards require that the purity of gold alloys be stated clearly and that the term used be consistent with the regulatory framework of the country of sale. Major auction houses — Christie's, Sotheby's, Bonhams — typically follow the convention of the country in which the sale is conducted, using "karat" in New York and "carat" in London, while always providing the millesimal fineness for clarity.

A Note on Diamond and Gem Pricing

Because the carat is the fundamental unit of gem valuation, price-per-carat figures are the standard metric for comparing stones of the same quality across different sizes. A ruby priced at USD 10,000 per carat weighing 3.50 ct has a total stone value of USD 35,000 — before any premium for the mounting. The non-linear relationship between carat weight and value (larger stones of fine quality command exponentially higher per-carat prices, a phenomenon sometimes called the "size premium") makes precise weight measurement essential. GIA-calibrated scales used in grading laboratories are accurate to 0.001 ct (one-tenth of a point), and weights are recorded before setting, as a mounted stone's weight cannot be measured directly.

No analogous per-unit pricing applies to gold purity: an 18 kt gold ring is not priced "per karat" but rather by the mass of metal at the prevailing spot price, adjusted for alloy composition and fabrication costs. The two systems thus operate in parallel but entirely separate commercial logics, reinforcing why their terminological separation is not merely conventional but functionally necessary.

Further Reading

• GIA — Carat Weight (4Cs)
• FTC — Revised Jewelry Guides
• CIBJO — The Blue Books (Precious Metals Standards)