Diamond prices are based on a how much carat weigh they have. For example, a diamond that weighs .33 carats and the list price is  for $3000 per carat. The way to calculate the  price is to multiply .33 x $3000 and the price you get is $990. However a diamond that weighs .66 carats goes into a much higher price category per carat and the price per carat would be $8000 per carat, and the bottom line would be $5280.

In the diamond business a price list called the Rapaport report or the Rap list is used which is based on specified carat weight ranges. The  lowest carat weight in each price range which defines the new price range for all carats larger than itself increase in price. These carat weights are can be defined "magic numbers." These special carat weight figures are as follows-

Seventy points(.70) through eighty nine(.89) ninety points (.90)through ninety nine (.99). From a carat on the price ranges change every half a carat 1.49- 1.99 and so on.

Since The prices change dramatically every time the carat range changes there is a very high demand on these diamonds whose carat weight is just under the magic numbers (for example ..69 points or .88 points). Diamonds with such weights are very hard to find because most cutters would much rather give up on a little of precision so that they can keep a diamond above the magic number. But its very ideal if you can find it, because the difference of .01 points is practically imposable to be seen and you'll be getting a diamond with the appearance of the next  carat range without paying the difference  The industry usually takes a little bit higher premiums for such diamonds, but you can still get a far better price than a diamond in the next carat range.