Carat (ct)
The carat is a unit of mass used exclusively for measuring gemstones and pearls, equal to exactly 200 milligrams or 0.2 grams. Abbreviated as "ct", it is the global standard in the jewellery and gemstone trade. When a diamond is described as "two carats", it weighs exactly 400 milligrams. The carat should not be confused with the karat (abbreviated "K" or "kt"), which measures the purity of gold alloys.
Definition
One metric carat equals exactly 200 milligrams (0.2 grams), 0.00705 ounces, or approximately 3.086 grains. Gemologists further subdivide the carat into 100 "points", so a 0.50 carat diamond can also be described as a 50-point stone. The metric carat is the only carat definition in use today; historical carats varied from about 187 to 216 mg depending on the country.
History
The word "carat" derives from the Greek keration, referring to the carob seed, which ancient gem traders supposedly used as a counterweight on balance scales because of its remarkably consistent mass (approximately 200 mg). In reality, carob seeds vary considerably, and the practice likely led to regional differences. By the early 20th century, the need for standardisation was clear. In 1907, the Fourth General Conference on Weights and Measures adopted the metric carat at exactly 200 milligrams, replacing dozens of slightly different national carats. This definition was quickly adopted worldwide and remains unchanged today.
Common Uses
The carat is the universal unit for trading diamonds, sapphires, rubies, emeralds, and other precious gemstones. Jewellery appraisals, insurance valuations, and auction house listings all specify gemstone weight in carats. The price per carat increases exponentially with size for high-quality diamonds, making precise carat measurement critical. Pearl weight is also expressed in carats or grains. In the watchmaking industry, jewel bearings are sometimes referenced by carat weight. The carat appears on every diamond grading certificate issued by laboratories such as GIA, AGS, and IGI.
Did You Know? Facts About Carat
- The Hope Diamond weighs 45.52 carats (9.104 grams) and is one of the most famous gemstones in the world.
- The Cullinan Diamond, the largest gem-quality rough diamond ever found, weighed 3,106 carats (621.2 grams) before cutting.
- A one-carat round brilliant diamond has a diameter of approximately 6.5 mm - slightly smaller than a pencil eraser.
- Diamond price per carat rises steeply at "magic sizes" like 0.50, 1.00, and 2.00 carats, because demand clusters around these round numbers.
- The word "carat" for gemstone weight and "karat" for gold purity both derive from the same Greek word, but they measure completely different things.