Irradiation

  • Nature of gemstone: Almost any gem material.
  • Material used: Radiations used can be produced in a gamma cell, a linear accelerator, cyclotron, synchrotrons and nuclear reactors or from atomic piles.
  • Possible effects: In general, when a gemstone is exposed to some form of radiation for a period of time, in almost all the cases:
    • There is a change of colour.
    • After irradiation the stone is subjected to heating to either obtain the desired colour change or to stabilize the colour center formed.
    • High energy radiations which affect the nucleus giving a good coloration can induce radioactivity in the stone.
  • Stones enhanced:
    • X-Rays: These produce a surface coloration which is short lived and tend to fade either immediately, or within a few hours or when opposed to strong heat. E.g.
      • Light yellow sapphire irradiated to orange yellow
      • Lilac pink kunzite irradiated to green
      • Colourless topaz irradiated to brown
    • Gamma Rays: These rays produce a uniform coloration in a stone without any localized heat being generated. These rays in general do not induce radioactivity and are thus safer. E.g.
      • Rock crystal irradiated to brown smoky quartz
      • Light pink tourmaline irradiated to dark pink/ red
      • Diamond irradiated to a uniform blue-green colour, on annealing, it becomes dark green.
    • Electrons (also known as cathode rays or beta rays): These radiations produce only a surface coloration and tend to generate localized heat. E.g.
      • Colourless topaz irradiated to blue
      • In diamonds, changes obtained are blue, blue-green, green (after annealing to about 800) type 1a - yellow / type 1b brownish red / type 2a - brown.
    • Neutrons: These are highly energetic uncharged particles which produce a uniform coloration. They induce radioactivity in the stone. E.g.
      • colourless topaz irradiated to blue (some common trade names are cobalt blue, sky blue, London blue, Swiss blue etc.)
      • Diamond changes obtained are green (after annealing to about 800), type 1a - yellow / type 1b - brownish red / type 2a - brown.
    • Other Radiations: such as high energy fast moving protons, deutrons alpha particles (helium ions) etc. are also used, though they may induce radioactivity.
  • Identification: The identification of irradiated stones is not possible with conventional gem testing equipment.
    • In the case of gemstones which have just been irradiated or those with residual radioactivity, it may be possible to detect signs of activity with a Geiger Mueller counter (measures radioactivity).
    • Structural imbalances which are caused as a result of irradiation can be identified in some cases with the help of spectroscopy - infrared and Raman spectrums. In irradiated diamonds the characteristic absorption spectrum for yellow to brown irradiated stones is 594nm, 498nm; Brown: 498nm, 504nm (both of equal intensity); Green: 504nm, 741nm.
    • In the case cyclotron treated diamonds, under magnification:
      • Those irradiated from the crown, show a dark ring within the girdle.
      • Those irradiated from the pavilion show an umbrella effect or a number of dark shadow lines around the culet.
      • Those irradiated through the side, there will be a zone of colour near the girdle.
    • A destructive technique would be to heat the specimens and observe the color change. This is not always reliable.

In most cases the stones are annealed and also allowed to cool before they are used for trading. The legal release levels set by authorities in different countries varies. Some are as follows:

Country Nanocuries/Gram
U.S.A. 1.0 nci/g
U.K. 2.7 nci/g
Asia 2.0 nci/g