A carat is equivalent to 200 milligrams. It is a measurement used to describe the mass of diamonds. A common mistake is to think that it describes the dimensions of a gem, but this is not the case.
In British English, carat is also used to describe the purity of gold. In American English, it is usually spelled karat. In both cases, 24 carat/karat gold refers to 100 percent pure gold.
The QuillBot Grammar Checker can help you ensure you are using the correct spelling of carat/karat.