» The whiter or more colorless a diamond (Color), the more beautiful is it. 
» The more purely a diamond (Clarity), the more valuable is it.
» The better the cross section (Cut) of a diamond, the more strongly is its brilliance, its fire.
» The more largely a diamond (Carat), the more rare is it.

It is the combination of these 4 criteria , which quality determines the quality and the brilliance of a diamond - and concomitantly its value.

COLOR:

The purest and rarest diamonds are virtually colorless. Although many diamonds may appear colorless to the untrained eye, the majority contain very slight traces of yellow, brown or gray.

A stone's beauty and value usually increase dramatically the more colorless it is. The spectrum ranges from light yellow, 'Z,' to totally colorless, 'D.'

CLARITY:

It is possible to find some imperfections in almost every diamond: tiny feathers and clouds - inclusions - that affect a diamond's value. Clarity grades range from 'I' (imperfect) to 'FL' (flawless).

A diamond is said to be 'flawless' if no inclusions or superficial blemishes can be seen under a standard 10-power jeweler's loupe. Truly flawless diamonds are extremely rare.

CUT:

More than any other quality, cut determines the fire and brilliance of a diamond. In order to maximize this fire and brilliance, the diamond cutter must place each of the stone's facets and angles in exact geometric relation to one another. On a classic round brilliant-cut diamond, for instance, fifty-seven or fifty-eight facets must be precisely aligned.

The cut of a diamond helps determine its brilliance.

CARAT:

Carat is the gemologist's standard measure of a diamond's weight. But size alone is almost meaningless unless you consider Cut, Clarity and Color. However, size does increase the value of a good quality diamond for the simple reason that large stones are rarer than small ones. In one million mined stones, only one weights a carat or more?.