Gemstone Irradiation Treatment
The process of irradiation involves the exposure of a specimen to a form of radiation. These include rays, constituting a large part of the electro-magnetic spectrum, as well as various energetic particles. Each of these sources has advantages and disadvantages.
In order to understand how irradiation can change the color of a gemstone, ‘color center’ concept should be explained first.
In order to understand how irradiation can change the color of a gemstone, ‘color center’ concept should be explained first.
Color Centers
The majority of the irradiation-induced color changes have not been studied in sufficient detail for a full understanding of the processes at. Usually optical absorption-spectroscopy must be backed up by paramagnetic resonance and other sophisticated techniques to provide an unequivocal answer. However, even where such complete insight is lacking, the major outlines are clear. Most of these color changes involve a 'color center'. The characteristics of a color center include its production by irradiation (although other specialized techniques can sometimes be employed to produce the same result). Heating invariably produces the reverse change, although exposure to light or even merely sitting for some time in the dark may be sufficient to produce loss of color in some particularly unstable color centers. Invariably, a color center involves one electron missing from a normally occupied position, leading to a 'hole color-center', or the presence of one extra electron leading to an 'electron color-center'. If the electron is merely one involved in the variable valence of a transition element, then the term color center is not usually used.
Any material that can form a color center contains two types of precursors, which have the following characteristics. The 'hole-center precursor' is an atom, ion, molecule, impurity, or other defect which contains a pair of electrons, one of which can be ejected by energetic irradiation, leaving behind a ‘hole center’. ‘The electron-center precursor’ is a similar atom, ion, etc., which can ‘trap’ the electron ejected from the hole-center precursor to produce an ‘electron center’. The role of the irradiation is thus exciting an electron, to leave an atom (ion, molecule, impurity, or other defect) forming simultaneously a hole center and an electron center. Almost all materials have hole-center precursors however, if there is no electron-center precursor, the electron displaced by irradiation immediately returns to its original place and the material remain unchanged.
Any material that can form a color center contains two types of precursors, which have the following characteristics. The 'hole-center precursor' is an atom, ion, molecule, impurity, or other defect which contains a pair of electrons, one of which can be ejected by energetic irradiation, leaving behind a ‘hole center’. ‘The electron-center precursor’ is a similar atom, ion, etc., which can ‘trap’ the electron ejected from the hole-center precursor to produce an ‘electron center’. The role of the irradiation is thus exciting an electron, to leave an atom (ion, molecule, impurity, or other defect) forming simultaneously a hole center and an electron center. Almost all materials have hole-center precursors however, if there is no electron-center precursor, the electron displaced by irradiation immediately returns to its original place and the material remain unchanged.
An unpaired electron can become excited by absorbing energy from a white-light beam, thereby removing part of the spectrum and producing color. Either or both of the hole- and electron-centers can do this. If it is only the hole center that absorbs visible light, a hole color-center is resent; if it is only the electron center, an electron color-center is present.
If light or heat now liberates the trapped electron from the electron center so that it can return to the original position, then the original state is restored and the color is lost, a process called fading or bleaching. If the trap is weak or ‘shallow’, even room temperature in the dark may be able to supply enough energy (about 0.1 eV) to release the trapped electron. With a trap a little stronger or ‘deeper’, it may require the 1 to 3 eV of visible light to produce fading, as in Maxixe beryl and unstable irradiated yellow sapphire or brown topaz.
A deep trap may hold the electron so strongly that the alteration is perfectly stable to light, as in smoky quartz, amethyst, blue topaz, or stable brown topaz; heating to as high a temperature as 500°C may then be required for bleaching. Such bleaching requires time; the higher the temperature, the more rapidly does it occur.
If light or heat now liberates the trapped electron from the electron center so that it can return to the original position, then the original state is restored and the color is lost, a process called fading or bleaching. If the trap is weak or ‘shallow’, even room temperature in the dark may be able to supply enough energy (about 0.1 eV) to release the trapped electron. With a trap a little stronger or ‘deeper’, it may require the 1 to 3 eV of visible light to produce fading, as in Maxixe beryl and unstable irradiated yellow sapphire or brown topaz.
A deep trap may hold the electron so strongly that the alteration is perfectly stable to light, as in smoky quartz, amethyst, blue topaz, or stable brown topaz; heating to as high a temperature as 500°C may then be required for bleaching. Such bleaching requires time; the higher the temperature, the more rapidly does it occur.
Major changes of selected gemstones on irradiation treatment table
Material | Changes |
---|---|
Beryl, aquamarine | Colorless to yellow, blue to green, pale colors to deep blue Maxixe |
Corundum | Colorless yellow, pink to padparadscha |
Diamond | Colorless or pale colors to blue, green, black, yellow, brown, pink, or red |
Pearl | Darken to gray, brown, blue, or black |
Quartz | Colorless, yellow, or pale green to smoky |
Tourmaline | colorless or pale colors to yellow, brown, pink, red or bicolor green-red; blue to purple |
Zircon | Colorless to brown or reddish |
Topaz | Colorless to yellow, orange, brown or blue |
Reference: Gemstone Treatment by Kurt Nassau, Butterworths, 1984.