When diamonds are formed naturally, they absorb natural gases such as Nitrogen which when reflected in the diamond can take on a slight coloured hue to the naked eye. The rarest and most desirable diamonds are those that are as close to colourless as possible. Most have yellow or brown tints, and subtle differences in colour can dramatically affect a diamond’s value, regardless of other factors.
Diamonds are graded for colour from D to Z, with grades D-F being considered colourless, G to J near colourless, K to M faint, N-R very light and S-Z light. At Harper Tait we only use diamonds graded D to F: that is, those that are officially rated as colourless. These grades are set and approved by the GIA (Gemological Institute of America).
Another key consideration is fluorescence. Roughly 35% of all diamonds emit a fluorescent glow when exposed to ultra-violet (UV) radiation, for instance from sunlight or fluorescent tube lights. The most common colour for this fluorescence is blue, and if a diamond has a yellow tint, then a blue fluorescence can cancel this out. A strong fluorescence can also give a diamond a cloudy appearance which reduces its worth.