“Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation. However, when the absorbed electromagnetic radiation is intense, it is possible for one electron to absorb two photons; this two-photon absorption can lead to emission of radiation having a shorter wavelength than the absorbed radiation. The emitted radiation may also be of the same wavelength as the absorbed radiation, termed "resonance fluorescence.” The most striking examples of fluorescence occur when the absorbed radiation is in the ultraviolet region of the spectrum, and thus invisible to the human eye and the emitted light is in the visible region.” (Wikipedia)
In the diamond business, we are concerned with fluorescence because if affects the value of the stone. How much it affects the value depends upon the size of the diamond, the color, clarity, shape, and the degree or strength of fluorescence The labs grade fluorescence as FAINT, MEDIUM, STRONG, and VERY STRONG. The strength grades are often accompanied by the color of the fluorescence. Ex: STRONG BLUE.
In addition, sometimes stones that fluoresce give off an oily or oil on water appearance to the diamond or they produce a cloudy or milky effect. The more pronounced the effect, the less value for the stone. In polished diamonds, the effect is often seen easily. For stones that have a G.I.A. certificate or any other lab certificate, the fluorescence is detailed in the certificate. For rough diamonds, fluorescence is often overlooked or is difficult to see with just your eye. However, with the use of a long or short wave light source it becomes easily visible and the strength of fluorescence can be accurately estimated.
In the rough diamond business, few institutional buyers and sellers have a real understanding of fluorescence. An experienced buyer or seller can use fluorescence, or the lack thereof, as a tool to negotiate better pricing.
It is interesting to note that fluorescence is not always detrimental to the value of a diamond. For diamonds of I-K color, fluorescence can actually give the diamond a whiter or blue-white appearance. If the stones do not milk or oil, then fluorescence will add value to these stones.
Fluorescence cannot always be seen. I have seen stones with VERY STRONG fluorescence that looked great with no oil or milking. In fact, fluorescent diamonds were once referred to as ‘blue-white’ diamonds.
It was not until the Japanese began buying fine quality diamonds in the 1990’s that fluorescence became an issue.
If you are a buyer or seller of rough or polished diamonds then fluorescence must be addressed in the pricing of any diamond or parcel of diamonds. If you do not understand or have the experience to calculate the affect fluorescence has on the diamonds you wish to buy or sell, then you would be wise to utilize the services of a rough diamond gemologist.
Louis Pearl G.G.
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