Tuesday, July 1, 2008

Luminescence

Most of the light effects discussed so far result from reactions between gemstones and visible light. However, certain gemstones do react to the stimulus of radiation which is beyond the limits of visible light. The most striking effects come from subjecting gemstones to short wavelengths just beyond the violet limits of the light spectrum. This ultraviolet light, or the even shorter X-rays, is absorbed and then given off again as longer and often visible wavelengths, according to a law discovered by Sir George G. Stokes in 1852. Thus, ruby when exposed to an ultraviolet light source will glow like a dull red coal, and some diamond rings may assume an eerie, bluish luminescence. The phenomenon is named "fluorescence" after the mineral fluorite in which it was first studied.
Occasionally, the re-emission of this changed shortwave radiation is delayed by the mineral. This phenomenon is known as "phosphorescence." The famous Hope Diamond, when exposed to strong ultraviolet radiation, produces little fluorescent reaction. However, it is startling to see the stone, once the exciting shortwave radiation is removed, glowing with a brilliant scarlet, delayed phosphorescence.
Luminescent effects in gemstones are not particularly important except as identification aids. Unfortunately, a milky blue fluorescence, when it occurs in some diamonds used for diamond bridal engagement rings under sunlight or strong incandescent light, can detract considerably from brilliance and value.

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