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Gemstones are valuable because they are a rare and desirable commodity. They also represent time and labour spent on cutting and presentation.
Because of their value and portability gems have often been used instead of money. Traditionally, their size is given in carats, which is a term of weight (1 carat = 0.2 gram).
A stone's value is determined by its quality and size. This is stated, as a rate per carat. The rate per carat is then multiplied by the actual weight of the stone to give a selling price. Large or special stones have an additional premium value.
Because synthetic gemstones can be produced to order in a laboratory, their value is much less than that of their natural counterpart.