What are different carat qualities of gold?

A carat is one fifth of a gram when measuring the weight of precious stones, When applied to gold it is the weight proportion of the alloy where 24 carats is the total. So 24 carat gold is pure 100% gold. Anything less is an alloy with some other metals (usually copper, silver, or nickel but many other metals can be mixed in).

Gold has such a high density (19.4) that if you mix equal volumes of copper (density 7.2) and gold you get 18 carat gold.