Overview

Carat has two related but distinct meanings in jewelry and gemology. In one sense, the carat (abbreviated ct) is a unit of mass used for gemstones and pearls. In another sense—often spelled karat (abbreviated K or kt), especially in North America—it denotes the purity of gold as a fraction of 24 parts. The two uses are separate: one measures weight, the other measures composition.

Carat as gemstone weight

The metric carat is the standard unit for weighing gemstones: 1 carat = 200 milligrams (0.2 grams). Gem weights are frequently expressed in decimal carats (for example, 0.75 ct) or in points, where 1 carat = 100 points so that a 50‑point stone equals 0.50 ct. Carat weight is one of the primary factors affecting a gemstone’s appearance and value; larger stones are rarer and often sell for a higher price per carat.

Karat as gold purity

The karat system divides a metal into 24 parts to express how much of it is pure gold. Pure gold is 24‑karat (24K). To find the gold percentage, divide the karat number by 24 and multiply by 100: for example, 18K gold is 18/24 = 75% gold, with the remainder usually copper, silver or other alloys added to increase hardness or change color.

History and terminology

The word carat derives from ancient terms for the carob seed, which were once used as weight references. In the early 20th century the metric carat was standardized to 200 mg to remove regional differences. Spelling varies by region: "karat" is widely used in the United States and Canada when referring to gold purity, while "carat" is commonly used internationally for gemstone weight and sometimes for gold as well.

Practical notes and distinctions

  • Abbreviations: carat = ct; karat often shown as K or kt.
  • Weight vs. quality: carat measures mass, not cut, color or clarity—those factors are also crucial to a gem’s value.
  • Purchasing: watch for decimal rounding in carat weights and hallmarks that state karatage for gold items; legal marking rules vary by country.

Understanding the difference between carat (weight) and karat (purity) helps buyers and collectors evaluate gemstones and precious metal jewelry more accurately, and prevents confusion caused by similar-sounding terms.