Carat (ct), usually abbreviated as Ct, has been used as an internationally recognized unit of measurement for gemstones since 1907. CT is the quality unit of jade, diamond and other precious stones, and it is the purity ratio of precious metals. International standard 4C for evaluating diamonds. 4C refers to color, clarity, weight and cut.

The word "Kela" comes from the Greek word "Kela", which refers to carobseed, a plant widely spread from East Asia to the Middle East. Because its fruit is said to have almost the same weight, and the weight of diamond is the most easily measured feature in 4c, the early carob tree was used as the weight unit of jewelry and precious metals. One carat is equivalent to the weight of a small horn tree seed.