The effect noticed in some substances of giving out the visible light when they are rubbed or scratched (Triboluminescence) or when they are irradiated with invisible electromagnetic 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 infrared 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 beam of electrons |
| Electroluminescence | A Form of luminescence produced by passing an electric current through them. Natural blue diamonds can be separated from artificially colored blue diamonds using this technique since the former are semi-conductors while the latter are not. |
Two methods used in Gemology to produce luminescence are:
- Ultraviolet Light
- X-rays
Ultraviolet Light:
The invisible rays, in the wavelength beyond the visible violet, having a range of between 100 and 380 nanometers.
| Longwave | 380 to 300 nanometers |
| Shortwave | 300 to 200 nanometers |
Fluorescence
| 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 |
X-Rays
Electromagnetic radiations of extremely short wavelengths falling between the ultra-violet and gamma rays of the electromagnetic spectrum. Effects seen under X-rays are less variable than those under U.V light and are therefore considered more conclusive.
Uses:
- Used in the separation of Diamonds from its host rock. All diamonds fluoresce a chalky-blue when exposed to X-Rays.
- Used to distinguish Natural from Cultured Pearls.
- Used to separate awkward Synthetic Rubies from Natural Rubies. Most Synthetic Rubies phosphoresce under X-Rays while Naturals do not.