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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:

  1. Ultra-violet Light
  2. X-Rays

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


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.


  1. Used in the separation of Diamonds from its host rock. All diamonds fluoresce a chalky-blue when exposed to X-Rays.
  2. 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.