Diamonds can assume almost all colours, due to impurities or structural defects: yellow and brown are the most common. Black diamonds contain numerous inclusions that give the gem its dark look.
When the colour is quite saturated in yellow or brown diamonds, the stone can be defined by the gemmologist as a fancy color diamond, otherwise it can be classified for colour according to the normal scale of colour of white diamonds.
The Gemmological Institute of America (GIA) classifies yellow and brown diamonds of low saturation as diamonds on the normal colour scale, and applies a graded scale from “D” (exceptional white) to “Z” (clear yellow.) The GIA classifies diamonds that have more colour than a “Z” as fancy color, together with those that have colours that are different from yellow or brown. The rarest colour diamonds are red, which never reach notable sizes, followed by intense green and blue.