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The Collaborative International Dictionary of English v.0.48 (gcide)

Carat \Car"at\ (k[a^]r"[a^]t), n. [F. carat (cf. It. carato,
OPg. quirate, Pg. & Sp. quilate), Ar. q[imac]r[=a]t bean or
pea shell, a weight of four grains, a carat, fr. Gr.
kera`tion a little horn, the fruit of the carob tree, a
weight, a carat. See {Horn}.]
1. The weight by which precious stones and pearls are
weighed.
[1913 Webster]

Note: The carat equals three and one fifth grains Troy, and
is divided into four grains, sometimes called carat
grains. Diamonds and other precious stones are
estimated by carats and fractions of carats, and
pearls, usually, by carat grains. --Tiffany.
[1913 Webster]

2. A twenty-fourth part; -- a term used in estimating the
proportionate fineness of gold.
[1913 Webster]

Note: A mass of metal is said to be so many carats fine,
according to the number of twenty-fourths of pure gold
which it contains; as, 22 carats fine (goldsmith's
standard) = 22 parts of gold, 1 of copper, and 1 of
silver.
[1913 Webster]