The weights of gemstones are usually measured in carats (ct.), but grams or kilograms may be used for large parcels of rough material or carvings.

The word "carat" is taken from the carob seed, which was used in ancient times to weigh gemstones. One carat is equal to 0.2 grams, so one gram would be equivalent to five carats. However, this unit should not be confused with its homonym, ‘karat’ (K or kt.), which is used to define the purity of gold.

For very small gemstones, especially diamonds, the ‘point’ system is sometimes employed, with 100 points being equivalent to one carat. This system avoids small numbers being used, as a 0.1 carat stone can instead be referred to as “ten points”.