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.

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