The weight of a diamond is, for historical reasons, not determined in milligram, but rather in carat.

The definition “Carat” is traced back to the seed of the carob tree (Ceratonia siliqua) which weighs about 200 mg. Therefore metric carat is determined as 1/5 of a gram or 200 mg. Today the carat weight is ascertained by an electronic carat scale, and the weight is stated with at least two decimal places (i.e. hundred parts of a carat).