Karat is the way in which we measure the purity of gold. Pure gold that is not mixed with any other metal is 24 karats. When pure gold is mixed with another metal the karat number is reduced to indicate that it is no longer pure gold. For example, 18 karat gold means that there is 75% pure gold and 25% of another metal mixed with the pure gold. Both yellow and white gold can vary in the purity of gold but are typically 18 or 14 karats for jewellery.