The purity of finished gold (e.g., jewelry) is often related in karats. A karat represents a 1/24th unit of the metal present in the alloy. Therefore, 24-karat gold is the purest (all 24 units are composed of gold) and is rated as 99.5% or higher pure gold. On the other hand, 18-karat gold is 75% gold and 25% other alloy metal such as tin or copper.