This is the phenomenon that allows some gemstones to glow.
Throughout history, there have been stories about magical gemstones that somehow had the ability to glow. While in the ark, Noah was said to have provided light by hanging Garnets. One of the early names for Ruby, “Carbuncle”, related to its ability to glow. Even the legendary writer of “Natural History”, Pliny the Elder (AD23 to AD79), wrote about curious luminous gems. Today we know this mystical ability of certain gems to glow as ‘fluorescence’.
Fluorescence produces vivid colours when an ultraviolet light source is directed at a gemstone. As we have discussed under the heading “colour”, visible light is broken down into the colours of the rainbow, all of which have wavelengths that are visible to the naked eye. Ultraviolet light however, has a short wavelength (less than 400nm) and is not visible to the naked eye. The ultraviolet light that the sun emits, although not visible, is what causes sunburn. When different gems are exposed to ultraviolet light, they behave differently. Although none get sunburnt, some are affected by the light, and while the source is present give off a strong glow. Burmese Ruby for example,i becomes intensely bright when seen in direct sunlight. Put simplistically, the cause of this effect is due to the fact that ultraviolet light is more energetic than visible light, boosting the energy of certain electrons, causing them to glow.
In 1852, Professor George Stokes - in a poorly lit room - allowed a small amount of sunlight through a small hole in a window blind to fall onto the surface of a colourless Fluorite gem. He noted how the ultraviolet light enhanced the appearance of the mineral. The word fluorescence yet again demonstrates how many words in the English language are derived from the colour and optical properties of gemstones.
If you ever receive a laboratory report with a gemstone, often they will state whether the gem fluoresces. The effect can be seen in about 30% of Diamonds which fluoresce blue under ultra violet light. A Diamond that demonstrates strong fluorescence tends to have a whiter colour than one which does not.
The term should not be confused with phosphorescence. Phosphorescent gems are those that retain their luminosity within the gem after the removal of the ultraviolet light. For example, similar to the phosphorescent arms used in some watches, when gems such as Kunzite are taken into the shade after being exposed to ultraviolet light, they may continue to glow for several minutes.
Now if we want to get incredibly technical, there are two more events worthy of a mention. Triboluminescence is the emission of light as a result of friction. Certain Feldspars and occasionally Diamonds and Quartz possess this ability. Other minerals have the ability to glow when they are heated, which is known as thermoluminescence. Apatite and Fluorite can possess this ability.
