The weight of a diamond is generally given in carats. The term carat originated in ancient times when gemstones were weighted against the carob bean. Each bean weighed about one carat. In 1913, carat weight was standardized internationally and adapted to the metric system. One carat equals 0.2 grams – a little more than 0.007 ounce. In other words, it takes 142 carats to equal 1 ounce.

Two terms, carat and karat are often confused. In the US, karat refers to the fineness of gold alloys (pure gold is 24 karat; 14 karat is 14 parts gold and 10 parts other metals) and carat refers to gem weights.

The weight of small diamonds is frequently expressed in points, with one point equaling 0.01 carats. For example, five points is a short way of saying 5/100 of a carat.

Note that an increase in carat weight does not produce the same increase in millimeter diameter. For example, there is a 25% increase in carat weight form 1.00 carats to 1.25 carats but less than 8% increase in diameter (6.5 to 7.0mm). This concept, along with the increased price per carat, explains why prices increase dramatically in order to get noticeably bigger millimeter size.