Why does this word cause so much confusion? Many people read carat as a measure of how big a stone is, but it measures weight. One carat is 200 milligrams, or one fifth of a gram, and each carat is split into 100 parts called points 1. Gold purity uses a similar-sounding word (spelled karat in some countries and carat in others), which measures how fine (pure) the gold is, not weight 2. Depending on the country, gold purity is shortened to ct or K, as in 18ct (18K) gold, so check whether a label describes the stone or the metal (see 9ct vs 14ct vs 18ct gold).
Carat: weight and size
Pictures: one AI-generated illustration of a diamond, adjusted for each grade. They show how the grades differ; they are not a grading of any stone.
Why can two one-carat gems look so different in size? Each material has its own specific gravity, which is its weight compared with the same volume of water. Diamond is 3.52 and ruby is 4.0, so a one-carat diamond is larger than a one-carat ruby 1. Emerald is lighter at 2.72, so a 1.00 ct emerald is larger than a 1.00 ct diamond 6.
Why is carat not a reliable guide, even between two diamonds? The way a stone is cut decides where its weight sits. A diamond that is too deep, or has a thick girdle (the thin outer edge of the stone), hides weight where you cannot see it from above. A poorly cut 1.20 ct diamond may look the same size as a well-cut 1.00 ct diamond, so judging size by carat alone can mislead you 2. The Carat to Millimetre Estimator can help you compare weight with visible size.
Example: Picture a 1.00 ct diamond and a 1.00 ct emerald. Each one weighs 200 milligrams 4. The emerald is still the larger stone, because emerald has a lower specific gravity than diamond 6. How big each one looks in a ring also depends on how well it is cut 2.