Gemmology
Luminescence
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).
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Fluorescence |
produced by exposure to invisible radiations, such as ultra-violet light or x-rays. |
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Phosphorescence |
the afterglow of fluorescence when the stimulating radiations have been shut off. |
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Thermoluminescence |
a secondary light generated by certain substances when heated with invisible infra-red rays. |
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Triboluminescence |
the phenomenon exhibited by certain minerals when they are rubbed or scratched of exhibiting luminosity. |
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Cathodoluminescence |
fluorescent effect displayed by some materials when they are bombarded with a been of electrons |
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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
- X-Rays
Ultra-violet Light:
The invisible rays, in the wavelength beyond the visible violet, having a range of between 100 and 380 nanometres.
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Longwave |
380 to 300 nanometres |
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Shortwave |
300 to 200 nanometres |
Fluorescence
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Paste (Glass) |
Inert |
Bright pale blue or green |
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Natural Blue Sapphire |
Inert (due to Iron) |
Inert |
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Synthetic Blue Sapphire |
Inert |
Greenish-blue due to Titanium |
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Natural Blue Spinel |
Inert |
Inert |
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Synthetic Blue Spinel |
Red (due to cobalt) |
Bluish-white |
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Cubic Zirconia |
Mustard yellow |
Less pronounced |
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Natural Purple Sapphire |
Inert |
Inert |
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Synthetic Purple Sapphire |
Red |
Bluish-white |
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Natural Ruby |
Red |
Red |
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Synthetic Ruby |
More intense red |
More intense red |
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Garnet |
Inert |
Inert |
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Natural Emerald |
Green to Red |
Inert |
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Synthetic Emerald |
Strong red glow |
Strong red |
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Diamond |
Sky-blue, blue, green to yellow. Stones that fluoresce blue have a yellow afterglow |
less pronounced |
X-Rays
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 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.
