The value of any gemstone depends on one or more of four attributes: cut, color, clarity and size. Since large stones are rarer than small ones, the value of a gemstone rises exponentially with its carat weight.
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Rubies are a relatively rare gemstone. Burmese rubies command the highest price. Stones that are recut or even exported before cutting are worth more per carat than native-cut rubies. According to the International Gem Society, clean, top-color gems command a retail price of $1,000 to $3,000 per carat.
In general, the deeper and more vivid an emerald's color, the higher its value per carat. Most emeralds have small fissures called inclusions; the fewer the inclusions, the more expensive the emerald. A large, deep-green emerald with relatively few inclusions and little secondary coloration may fetch tens of thousands of dollars per carat.
Diamonds have steadily increased in value and show no signs of slowing. The pricing of diamonds can range anywhere from $1,000 to $40,000 per carat, depending on clarity, cut and color.