Karat vs Carat: What's the Difference?

Karat (K or KT) measures gold purity — 24K is pure gold, 18K is 75% gold. Carat (ct) measures gemstone weight — one carat equals 0.2 grams. Same sound, different quantities, different materials.

Gold is rarely used in its pure state because 24K is too soft for everyday jewellery. Jewellers mix it with copper, silver, or palladium to add hardness and change its colour. The karat number tells you how many parts out of 24 are pure gold: 18K is 18/24 (75%), 14K is 14/24 (58.5%), and 10K is 10/24 (41.7%).

The letter K or KT on a hallmark always refers to gold purity. In some countries, equivalent millesimal fineness stamps are used instead: 750 for 18K, 585 for 14K, and 375 for 9K.

For gemstones, carat is a unit of mass. One metric carat equals exactly 0.200 grams (200 milligrams). The carat was standardised internationally in 1907 to replace a patchwork of regional seed-based reference weights. Larger stones are rarer, so price per carat rises steeply with size.

Diamond pricing jumps at milestone weights like 0.50 ct, 1.00 ct, and 2.00 ct because demand clusters around round numbers. A 0.99 ct stone may cost meaningfully less than a 1.00 ct stone of identical quality.

Both words descend from the same root: the Greek kerátion (κεράτιον) and Arabic qīrāṭ, referring to the carob seed once used as a counterweight on scales. Over centuries the term split into two separate measurement standards — purity for gold and mass for gems — but the spelling in English remains almost identical, which is the source of the persistent confusion.