Carrot, the vegetable, has nothing to do with carat, which comes from a different kind of plant. Carat is the Arabic word for the seed or bean of the carob tree of the Mediterranean. It usually weighed so close to four grains* that ancient jewelers used to balance the beans on a scale opposite a diamond. Today a carat is 3,086 grains, and all precious stones are measured in carats, though the beans have of course fallen out of use.

*Today a grain is 64.79891 milligrams; in 1758 (and possibly earlier) a grain referred to the weight of a seed from the middle of an ear of barley.