This is the easiest criterion. The unit used to weigh diamonds is the carat, equivalent to 0.20 gram. Below one carat, the term hundredths is used. A diamond labelled 0.25 weighs 25 hundredths (or 25 points).
The word ‘carat’ comes from the carob seed, which was previously used to weigh stones.
The weight of a diamond determines its size and therefore its diameter.
Here are a few examples of round diamond diameters depending on their weight :