Cut:

Only a perfect cut reflects the incident light and creates the fire within the diamond. A diamond with an ideal cut deflects the light through the facettes causing a perfect reflection (brillance).
A diamond with a low cut looses the fire because the light goes though the diamond. If the cut is too high, the light beams escape through the sides instead of coming through the top.

Color:

An additional atribute is the color. Diamonds are graded using a colour scale that ranges from 'D' (completely colourless) to 'Z' (tinted yellow).
The higher the colour the brighter the diamond and that increases the value of the diamond.

Clarity:

The clarity is another important attribute. Most diamonds have small imperfections, called inclusions.The clarity grade depends on how many inclusions there are and how visible they are.
Inclusions can be crystalline and firm structures, punctual shapes, clouds, cracks, splits, feathers, as well as growth layers. The less inclusions a diamond has the "purer" is the stone and the higher the value. For more details see our diamond encyclopedia.

Carat:

The weight of a diamond is measured in carats. One carat equals 1/5th of a gram. A carat is divided in 100 points so that a diamond of 50 points equals 1/2 a carat. Large diamonds are much rarer and their value increases exponentially with size.