A chemically pure and structurally perfect diamond is perfectly transparent with no hue, or colour. However, in reality almost no gem-sized natural diamonds are absolutely perfect.
Letters of the alphabet ranging from
D-F colourless or finest white
G-J white or near colourless
K-M tinted white or faint yellow
N-R tinted or very light yellow
S-Z light yellow
Diamonds occur in a restricted variety of colours known as fancies; steel grey, white, blue, yellow, orange, red, green, pink to purple, brown, and black. Coloured diamonds contain interstitial impurities or structural defects that cause the colouration, whilst pure diamonds are perfectly transparent and colourless. Fancies have their own separate colour grading system.
Colour is typically the most important factor in determining value for gemstones. Every stone has its own preferred colour or range of colours and the more the colour varies from this standard, the less valuable the stone.
A stone's colour is determined by three factors: hue, tone and saturation.
Hue refers to the immediate impression of its colour, e.g. red or blue.
Tone defines the degree of lightness or darkness of the stone.
Saturation refers to the degree of purity of a hue.