Why are Fancy-Coloured Diamonds so Rare?

Fancy Colored Diamonds

Only one in a hundred thousand diamonds has a deep enough natural colour to qualify as a ‘fine, fancy-coloured diamond’.

Gemmologically, minute traces of various chemicals give rise to different colours in diamonds. Traces of nitrogen will give rise to yellow stones while blue diamonds have traces of boron.
Unlike sapphires and other gemstones, fancy-coloured diamonds possess a distinctive look not found in gemstones. A diamond has the ability to disperse each ray of reflected light into a rainbow of colour.

by Selwyn Kahn