Fluorescence in Diamonds

While doing some research into diamonds I started looking into the subject of fluorescence – I thought you might be interested to read the results.

Fluorescence in diamonds is considered rare. 65 percent of diamonds exhibit no fluorescence at all yet it is often mentioned on diamond certificates. So what is it?

Fluorescence is the glowing colour (usually blue) that appears when some gem-quality diamonds are exposed to long-wave ultraviolet light. As soon as the ultraviolet light source is removed, they stop fluorescing. It is a naturally-occurring optical phenomenon and is completely harmless, but for years it was thought that it was a negative attribute in a diamond.

In fact, it can actually be a good thing in diamonds of a lower colour grade (I or lower). Daylight is a source of ultra violet light, so if you look at a diamond of this quality with strong fluorescence the blue colour can cancel out the yellow body-colour, making it appear more colourless. This means that you can buy a stone of a lower colour quality than you would otherwise, for less money than you would pay for a higher quality gem.

The History- myths and facts

The myth about fluorescence being a bad thing in diamonds of higher colour grades was disproved by the Gemological Institute of America (GIA) when they conducted an extensive study of the affect of fluorescence in diamonds. They compared over 1000 stones, and the percentage of diamonds that it affected negatively was so small it was non-existent! It is known that good coloured diamonds with very strong blue fluorescence can display a cloudiness referred to as ‘overblue’ but it occurs so rarely that most jewellers will never encounter it in their life. The study also showed that the difference between varying levels of fluorescence was indistinguishable to most non-jewellery trade customers they asked, ie for most people buying jewellery, fluorescence had no visible effect on a diamond’s colour appearance or transparency.

Interestingly, blue diamonds like the famous Hope Diamond sometimes display an extremely rare and unusual red fluorescence, rather than the more normal blue. In the case of the Hope Diamond, this has probably added to the legend of its curse! This red fluorescence has been researched recently by Jeffrey Post (curator of the National Gem Collection and mineralogist at the Smithsonian’s National Museum of Natural History) and the results have helped provide a ‘fingerprint’ to identify individual blue diamonds.

Gradations of fluorescence on certificates range from None or Inert (meaning no fluorescence at all) through Faint or Negligible (a very slight glow of colour that is difficult to see under ultraviolet light) to Medium (a light glow of colour under ultraviolet light) Strong (a deep, even glow that is evident under ultraviolet light) and Very Strong (a very deep, even glow that is very evident under ultraviolet light)