Not to be confused with the "carat" unit signifying the purity of gold, diamond weight is expressed in carats. "Carat" is derived from "Keration", the Greek name for the pod of the carob tree, whose individual seeds weigh approximately 0.2g. For this reason they were used as units of weight for gemstones in the Roman city of Byzance.

Until 1907, the carat weight could vary according to where it was being used, but that year the metric unit of 0.2g being equal to one carat was fixed, each carat is divided into 100 'points'.

The Carob Tree