Brewer's: 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.
Shakespeare: Comedy of Errors, iv. 1.Here's the note How much your chain weighs to the utmost carat.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894