Almost all natural diamonds contain small traceable quantities of nitrogen atoms that displace the carbon atoms within the crystals lattice structure. It is these nitrogen atoms dispersed throughout the stone that give them their yellow appearance. The lower the amount of nitrogen present within the diamond the whiter it will be. In determining the colour grading of a white diamond the Gemological Institute of America uses a scale of D to Z.
D, E, F colourless
G, H, I, J near colourless
K, L, M faint yellow or brown
N, O, P, Q, R very light yellow or brown
S, T, U, V, W, X, Y, Z light yellow or brown
Fancy Light the start of fancy coloured diamonds
Fancy Vivid the highest saturation of colour