Gemstone Carat
A carat is a unit of mass used for gemstones, defined as exactly 0.2 grams (200 milligrams). A 5-carat diamond weighs 1 gram. The word comes from carob seeds, which were historically used for their remarkably consistent weight.
Gold 'Carat' Is Different
For gold, 'carat' (or 'karat' in the US) refers to purity, not weight. 24-carat gold is pure; 18-carat is 18/24 = 75% gold. This carat has no unit of mass at all — it's a ratio. The two meanings share a word but describe unrelated properties.
Quick Reference
| Carats (gem) | Grams |
|---|---|
| 1 | 0.2 |
| 5 | 1.0 |
| 10 | 2.0 |