What is the unit of carat which is used for measuring the weight of diamonds?

A species of tree known as carob grows in the coastal areas of Mediterranean Sea (e.g. in Egypt, Libya, Tunisia, France, and Spain). It reaches the height of about 15 meters. It bears about 30 centimeters long ponds containing five to fifteen seeds which are flat and hard like tamarind seeds. These seeds of carob tree are known as carat. (See the picture.) They contain about 50% sugar so in the time of drought when food is scarce poor people in North Africa eat these seeds in order to survive.

Years ago jewellers of Europe used carob seeds as tiny weights for weighing diamonds because of near uniformity in the weight of these seeds. The difference between weights of any two carob seeds is not more then two to five milligrams. (1 milligram = 0.001 gram). One carob seed weighs about 0.2 gram so it requires 5000 carats against one kilogram weight to make the pans of scale to balance. Nowadays carob seeds are not used to weigh diamonds but the jewellers still use the word carat. According to an international agreement made in 1913 diamonds are weighed against a tiny weight of 200 milligrams, i.e. 0.2 gram.

On the other hand the scale indicating purity of gold is also called carat. When no other metal or impurity is mixed in gold it is considered as 24 carat (100%) gold, whereas in 18 carat gold remaining 6 carats equivalent (24- 18=6 i.e. 25%) comprises of other metal or alloy mixed with gold.

Additional reading:
Carat (mass) (Wikipedia)
Carat (purity) (Wikipedia)

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