Fluorescence occurs in some diamonds when they are exposed to the concentrated radiation of a UV lamp. Submicroscopic structures in the diamonds cause them to emit a visible light, a fluorescence, which is commonly blue in colour. Despite the fact that fluorescence is used as an identifying characteristic rather than a grading factor in most laboratory quality reports, its presence in such reports is being used with increasing frequency to determine the pricing of a diamond. In most cases, prices are lowered when a colourless or near-colourless diamond fluoresces under UV light, due to a common perception that fluorescence has a negative effect on the appearance of diamonds. However, there are many who believe that fluorescence has no adverse effect; some even contend that it enhances colour appearance.
Diamonds that are said to be fluorescent contain particles that emit a visible (usually blue) glow when exposed to UV lighting. In rare cases, high levels of fluorescence can make stones appear milky or hazy, although for the most part fluorescence does not generally impact beauty or sparkle and can even make some lower colours (I, J, K, L, etc.) appear more colourless or white.
In Europe, before the age of diamond certification, the most highly regarded diamond colour was ‘blue white’. This term describes near-colourless to light yellow diamonds with a strong blue fluorescence, stones that were actively sought by merchants thanks to their appealing ‘ice’ effect.