The quality, value and cost of a diamond are determined by four points known as the "4 Cs"

Cut
The angles and proportions of the stone.

Colour
How little colour the diamond contains. The more colourless, the more valuable.

Clarity
Any flaws or impurities in the stone.  Flawless is the highest grade of diamond.

Carat
The stone's weight, which determines its size. Smaller diamonds can be more expensive than large if their cut, colour and clarity rate higher.