Karat is the American spelling of carat, but in real American life the two spellings have drifted apart. When Americans write karat, they almost always mean gold: how much of a ring or chain is pure gold. Pure gold is 24 karat. 18-karat gold is 18 parts gold out of 24, mixed with other metal, so it is less yellow and stands up better to daily wear; 14-karat and 10-karat are more mixed still, and cheaper. Shops often just print K, as in 18K. Though actually, the tidy line “American spelling of carat” hides a split: Americans keep carat for the weight of a diamond or other gem, and karat for the gold mix. British English uses carat for both, which is why learners mash them together. Careful American writing will not call a diamond a “2-karat stone.”