Overview

The carat is the international unit of mass commonly employed in the gemstone and pearl trade. It expresses the weight of individual stones and is one of the primary attributes used to describe and price gems. In everyday use the word "carat" refers specifically to mass and should not be confused with "karat," which denotes the purity of gold.

Definition and subdivisions

By modern definition, one carat is equal to 0.2 grams (200 milligrams). The carat is decimalized for convenience: a carat is divided into 100 equal points, so each point corresponds to 0.01 carat or 2 milligrams. Jewelers commonly state diamond weights to two decimal places (for example, 0.75 ct or 75 points).

Characteristics and measurement

  • Precision: Gem weights are usually measured with a balance that reads to hundredths of a carat for retail descriptions and often to thousandths in appraisal or laboratory contexts.
  • Notation: The abbreviation "ct" is widely used in catalogs and certificates.
  • Trade practice: Weights just below a rounding threshold (for example 0.99 ct) can affect price significantly; consequently, exact measurements and reported rounding rules matter in valuation.

History and standardization

The term originally derived from the carob seed, once thought to have a uniform weight used as a natural reference. Over time many regional systems and variable standards existed, which made international trade awkward. The modern metric carat — exactly 0.2 g — was adopted at the Fourth General Conference on Weights and Measures in 1907 and has been used internationally since then to provide a consistent basis for gem commerce and scientific work.

Uses, examples and significance

The carat is the principal weight unit for diamonds, colored gemstones and pearls. Because gemstone value frequently increases nonlinearly with size, the difference between, say, a 0.90 ct and a 1.00 ct stone can be substantial. A common way to express small differences is by points: a 0.50 ct stone can also be called a 50‑point stone. Laboratories and grading reports list carat weight alongside other characteristics such as cut, color and clarity.

Distinctions and notable facts

It is important to distinguish the mass-based carat from the homophone "karat", which measures gold purity. For background on units and measurement, see the concept of a unit of mass and general unit standardization discussions at international metrology references. For information specifically about gemstones and trade terminology, consult reputable gemological resources such as those provided by major laboratories and professional organizations specializing in gems.

Overall, the carat remains a compact, easy-to-use measure that links traditional gem trade practice with the precision of the metric system, supporting consistent valuation, certification and scientific description of gem materials.