The effect noticed in some substances of giving out visible light when they are rubbed or scratched (Triboluminescence) or when they are irradiated with invisible electro-magnetic radiations (Fluorescence, Phosphorescence or Thermoluminescence).
| Fluorescence | produced by exposure to invisible radiations, such as ultra-violet light or x-rays. |
| Phosphorescence | the afterglow of fluorescence when the stimulating radiations have been shut off. |
| Thermoluminescence | a secondary light generated by certain substances when heated with invisible infra-red rays. |
| Triboluminescence | the phenomenon exhibited by certain minerals when they are rubbed or scratched of exhibiting luminosity. |
| Cathodoluminescence | fluorescent effect displayed by some materials when they are bombarded with a been of electrons |
| Electroluminescence | form of luminescence produced by passing an electric current through them. Natural blue diamonds can be separated from artificially coloured blue diamonds using this technique since the former are semi-conductors while the latter are not. |
Two methods used in Gemmology to produce luminescence are:
Ultra-violet Light:
The invisible rays, in the wavelength beyond the visible violet, having a range of between 100 and 380 nanometres.
| Longwave | 380 to 300 nanometres |
| Shortwave | 300 to 200 nanometres |
| GEMSTONE | LONGWAVE | SHORTWAVE |
| Paste (Glass) | Inert | Bright pale blue or green |
| Natural Blue Sapphire | Inert (due to Iron) | Inert |
| Synthetic Blue Sapphire | Inert | Greenish-blue due to Titanium |
| Natural Blue Spinel | Inert | Inert |
| Synthetic Blue Spinel | Red (due to cobalt) | Bluish-white |
| Cubic Zirconia | Mustard yellow | Less pronounced |
| Natural Purple Sapphire | Inert | Inert |
| Synthetic Purple Sapphire | Red | Bluish-white |
| Natural Ruby | Red | Red |
| Synthetic Ruby | More intense red | More intense red |
| Garnet | Inert | Inert |
| Natural Emerald | Green to Red | Inert |
| Synthetic Emerald | Strong red glow | Strong red |
| Diamond | Sky-blue, blue, green to yellow. Stones that fluoresce blue have a yellow afterglow | less pronounced |
Electro-magnetic radiations of extremely short wavelengths falling between the ultra-violet and gamma rays of the electro-magnetic spectrum. Effects seen under X-rays are less variable than those under U.V light and are therefore considered more conclusive.
Uses:
Used to separate awkward Synthetic Rubies from Natural Rubies. Most Synthetic Rubies phosphoresce under X-Rays while Naturals do not.