The term carat (or karat) has a double meaning: carat is used as a measurement of weight for gemstones: one carat equal 1/5 gram. Carat is also used in countries around the world to indicate the amount of pure gold in a piece of gold jewelry. In the USA, when using the word to indicate gold content rather then gemstone weight, it is spelled with a K. The word karat (carat) is derived from the word for the fruit of the carob tree (Greek -keration), whose seeds were used in ancient times for weighing gems because of their uniformity. The pure gold Byzantine coin called the silidus weighed 24 karats, which 24 karats, which later became the mark used to indicate that something was pure gold. In some cultures, 24 -karat gold is required for certain jewelry pieces, but it's generally agreed that 24-karat, or pure gold, is too soft for jewelry use.

Let's say, for instance, pure gold is a pie divided into 24 equal slices. each karat -one part of the pie, so 24K would be 100 percent gold - or pure gold. In 18 karat gold, 18 slices will be pure gold and six will be another metal. In 12 karat, 12 slices will be pure gold and 12 another metal.