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A diamond's weight is measured 'carats', which is a small unit of measurement equal to 200 milligrams. The following diagram shows the relative size of various carat weights for a diamond cut to the same proportions:

Note: the diamonds illustrated are not shown actual size.

Two diamonds of equal Carat Weight may vary substantially in price due to their Cut, Color and Clarity. For example, a diamond of exceptionally high quality may sell for $20,000 per carat, while one of lesser quality may sell for $1,000 per carat. Diamond values also increase disproportionately as the size of the stone increases. In other words, a two-carat stone will not necessarily cost twice per carat than a one-carat stone. It could cost much more, since diamonds are rarer in larger sizes.

Cut is the only one of the 4Cs that is within the craftsman control. A diamond has facets that allow light enter to it, become refracted, and exit in a rainbow of colors. So it follows that a better cut diamond does a better job of dazzling us with its beauty as shower in the figure below:

As you can see, the diamond with the correct proportions does a more effective job of refracting light out the top of the stone to your eye. The less-than-ideal cut diamonds allow some light to become lost out the bottom.

An 'ideal cut' is a specific set of guidelines that delineate the proportions that give a diamond the highest amount of fire and brilliance and cause the light entering the diamond to be reflected and dispersed through the table (top), not through the sides or bottom. But most of all, an ideal cut diamond must be cut to bring out the stone's brilliance and fire, not retain the most weight from the rough cut stone.

Diamond.com maintains extremely stringent guidelines for diamond quality. All diamonds we offer must fall within specific ranges to be judged acceptable by our staff gemologists.

The most important thing to know about diamond color is: the less color a diamond has, the more valuable it is, all other factors being equal. Diamonds are found in nature in a wide range of colors, from completely colorless (the most desirable trait) to slightly yellow, to brown. So-called 'fancy color diamonds' come in more intense colors, like yellow and blue, but these are not graded on the same scale.

The diamond color grading system uses the letters of the alphabet from D through Z, with 'D' being colorless and therefore the rarest and most valuable, and 'Z' having the most color within the normal range, and being the least valuable, all other factors being equal. A diamond's color is determined by looking at it under controlled lighting and comparing them to the Gemological Institute of America's color scale, which is based on a set of diamonds of known color. Here is a diagram showing how a diamond's color is graded:

Clarity refers to the number, position and size of the inclusions that occur naturally inside diamonds. Fewer and less obvious the inclusions will produce a more valuable diamond. Here is an illustration that shows the clarity grading scale that has been established by the world's foremost authority on diamonds, the Gemological Institute of America (GIA). Note: Diamonds are shown under 10X magnification):