The value of a diamond is judged worldwide by four criteria – the so-called 4C. They were introduced by the Gemological Institute of America (GIA).
Cut
The cut determines how a diamond reflects light – its sparkle and fire. It's the only one of the four criteria shaped by human hands, and many regard it as the most important.
Color
Colour is graded on a scale from D (completely colourless) to Z (clearly tinted). The more colourless a white diamond, the rarer and usually the more valuable it is.
Clarity
Clarity describes how free the stone is of tiny inclusions – assessed under tenfold magnification with a loupe. Fewer and smaller inclusions usually mean more brilliance.
Carat
Weight is given in carats (1 carat = 0.2 grams, divided into 100 points). Larger diamonds are rarer, so the price rises disproportionately with weight. Only the interplay of all four Cs produces the value.