Many ornamental stones, gemstones, imitations or syntheses fluoresce under the light of ultraviolet lamps. UV lamps usually emit longwave UV light (366 nm wavelength) and short-wave UV light (254 nm wavelength). Individual characteristic colours or colour changes at different wavelengths allow stones to be distinguished from one another. A large number of gemstones, cut or rough, can be inspected simultaneously with irradiation by an UV lamp. Occasionally the UV behaviour of the stones under irradiation with the two different wavelengths is so characteristic that the UV lamp can also be used to directly determine stones. This procedure, however, requires experience.
Functions of UV light and luminescence
If minerals – in our case gemstones – or synthetic products are irradiated with ultraviolet light, different reactions can occur. Some samples do not react to UV radiation at all. In contrast, other samples exhibit a varying degree of glow in different colours in the dark. An afterglow is sometimes visible when the UV lamp is already switched off. The appearance of the glow under UV light is known as fluorescence, the afterglow as phosphorescence – collectively also called luminescence.
In the case of fluorescence, the electrons leave the higher energy level almost immediately. The glow only lasts for as long as the UV radiation is effective. In the case of phosphorescence by contrast, the higher energy level is not immediately left again. The energy is released more slowly here. This is why the luminescence continues for a moment when there is no more radiation affecting the gemstones.
Our exampleshows green and light blue synthetic spinels under short-wave UV light. In addition, a functional representation with the ground state and the excitation of an atom (upper circle) and the return of its initial position (lower circle), where energy is then released as heat and fluorescence radiation.
Do you want to read more?
If you are looking for more expert information, order the white paper “Gemstone examination with UV light”. The publication describes e.g. the following topics:
- Examination of gemstones, jewellery stones, imitations or syntheses with ultraviolet light
- The identification of stones with UV light
- Distinguishing synthetic spinel from natural aquamarine or blue topaz
- Fluorescence of aquamarine, topaz, tourmaline or peridot
- Fluorescence of ruby, alexandrite or the red spine
- Chromium contents and fluorescence
- Identification of manipulated, impregnated or reconstructed stones
- The characteristic appearance of diamonds under long-wave UV light
The whitepaper was produced in collaboration with Prof. Dr. Jochen Schlueter wrote. He is a graduate mineralogist and responsible for the display collection of the Mineralogical Museum in Hamburg.
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