Carat vs karat
Carat (with a c) is a unit of weight for gemstones, equal to 200 mg. Karat (with a k) is a unit of gold purity, equal to 1/24 of pure gold. The two words sound the same in English but mean different things — a "1 carat diamond" weighs 0.2 g, while "18 karat gold" is 75% pure gold.
Why 0.2 grams
The metric carat was standardised by the International Bureau of Weights and Measures in 1907 at exactly 0.2 g, replacing a long history of regionally different "carat" values that had ranged from 188 mg to 213 mg. Today every gem certificate worldwide uses the metric carat.
Points and grain
Diamond dealers also use points: 1 point = 1/100 carat = 2 mg. A "75-point" diamond is 0.75 ct. Pearls use the grain: 1 grain = 1/4 carat = 50 mg.
FAQ
Does a heavier diamond cost more? Yes, and disproportionately — a 2 ct stone of equal quality typically costs more than four 0.5 ct stones combined.
Are lab-grown carats the same? Yes. A carat is purely a measure of weight, regardless of origin.