Carat (ct)
A unit of weight for gemstones: one carat is 200 milligrams, divided into 100 points.
Easily confused with
How to read it
GIA defines the metric carat as 200 milligrams, subdivided into 100 points, and notes that carat measures weight, not size. The FTC’s jewelry guides use the same definition: a carat is 200 milligrams, and a point is one hundredth of a carat. GIA traces the word to carob seeds, which early traders used as counterweights.
Also searched as: carat weight, ct, diamond carat, points.
Grades describe a stone as a grading laboratory saw it. This page explains the grading scales; it doesn’t grade or value any stone.
Carat or karat?
Two different measures. For gemstones, carat is weight; for gold, the fineness is counted in karats, as in 14K. The UK’s Hallmarking Act spells both carat and draws the same line: for a precious stone, carat is used as a measure of weight, not of fineness.
More diamond grading
Sources
- Diamond Carat Weight (GIA 4Cs), Gemological Institute of America
- Guides for the Jewelry, Precious Metals, and Pewter Industries (16 CFR Part 23), 16 CFR Part 23 (1 January 2021 edition), Federal Trade Commission, published by the U.S. Government Publishing Office, § 23.18, note
- Hallmarking Act 1973, Schedule 1, Hallmarking Act 1973 (c. 43), Sch. 1, Pt. III, legislation.gov.uk (The National Archives), Sch. 1, Pt. III
Last reviewed September 27, 2026 by Rory Hansen. How we research