What is a karat used to measure?

The karat (carat in Britain) is used for two things.

(1) as the weight of a gem such as a diamond. 1 karat is 0.2 gram (gramme).

(2) as the purity of gold. 24 karat gold is 100% gold. 18 karat gold is 75% (18/24) gold. [Gold is combined with silver and/ or copper when not 24 karat]