E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Carat of Gold.
So called from the carat bean, or seed of the locust tree, formerly employed in weighing gold and silver. Hence the expressions 22 carats fine, 18 carats fine, etc., meaning that out of 24 parts, 22 or 18 are gold, and the rest alloy.