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.