Gem Identification Instrument 253.7nm / 365nm
The types of UV light used in testing are long wave (LWUV) with a principle wavelength of 365nm, and short wave (SWUV) with a principle wavelength of 254nm.
Different testing equipment ranges from UV keyrings (typically LWUV) to a UV viewing cabinet.
When using UV light to test gemstones, it is important to remember that any exposure to UV light can damage your eyes, but particularly use caution when using SWUV as it is more dangerous than LWUV.
Always wear protective UV goggles or ensure that your UV cabinet is installed with an eyepiece that filters out UV light.
In gemmology the term photoluminescence, which is the emission of a cold, visible light when a gem material becomes stimulated by light of a shorter wavelength. Two examples are fluorescence and phosphorescence.
Fluorescence occurs when a gem material is illuminated by radiation of shorter wavelengths with higher energy. The visible light emitted stops when the source of illumination is turned off.
Phosphorescence, on the other hand, is a visible light that is emitted by a gem material after the original source of exciting radiation has been switched off.
A famous example is the blue Hope Diamond, which glows a bright red for several minutes after being stimulated by short wave UV light it is said to strongly phosphoresce.
Both fluorescence and phosphorescence vary very strong to weak.
If a material does not either fluoresce or phosphoresce, it is considered inert.
Some gemstones have a characteristic or, very rarely, a diagnostic reaction to UV light.
One gemstone that notably both fluoresces and phosphoresces is a diamond, which typically fluoresces blue in longwave UV light and then phosphoresces yellow.
Fluorescence can indicate or confirm the identity of a stone.