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Gemstone irradiation
Process to enhance a gemstone's optical properties

Gemstone irradiation is a technique used in the jewelry industry where a gemstone is exposed to high levels of ionizing radiation to enhance its optical properties. This radiation alters the crystal structure, changing the stone’s color or reducing visible inclusions. The irradiation is performed using a particle accelerator for electron bombardment, a gamma ray source like cobalt-60, or a nuclear reactor for neutron bombardment. While this process enables creation of rare gemstone colors, treatment in nuclear reactors can leave residual radioactivity, prompting regulations due to potential health risks.

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Radioactivity and regulations

See also: Induced radioactivity and Neutron activation

The term irradiation broadly refers to the exposure of matter to subatomic particles or electromagnetic radiation across the entire spectrum, which includes—in order of increasing frequency and decreasing wavelength—infrared, visible light, ultraviolet, X-rays, and gamma rays.6 Certain natural gemstone colors, such as blue-to-green colors in diamonds7 or red colors in zircon,8 are caused by exposure to natural radiation in the earth, which is usually alpha or beta particles.9 The limited penetrating ability of these particles result in partial coloring of the gemstone's surface.10 Only high-energy radiation such as gamma rays or neutrons can produce fully saturated body colors,11 and the sources of these types of radiation are rare in nature, which necessitates the artificial treatment in jewelry industry. The process, particularly when done in a nuclear reactor for neutron bombardment, can make gemstones radioactive.1213 Neutrons penetrate the gemstones easily and may cause visually pleasing uniform coloration, but also penetrate into the atomic nucleus and cause the excited nucleus to decay, thereby inducing radioactivity.14 So neutron-treated gemstones are set aside afterward for a couple of months to several years to allow the residual radioactivity to decay,1516 until they reach a safe level of less than 1 nanocurie per gram (37 Bq/g) to 2.7 nanocuries per gram (100 Bq/g) depending on the country.17

The first documented artificially irradiated gemstone was created by English chemist William Crookes in 1905 by burying a colorless diamond in powdered radium bromide.1819 After having been kept there for 16 months, the diamond became olive green.20 This method produces a dangerous degree of long-term radioactive contamination and is no longer in use.21 Some of these radium-treated diamonds—which are still occasionally put on sale and can be detected by particle detectors such as the Geiger counter,22 the scintillation counter,23 or the semiconductor detector24—are so high in radiation emission that they may darken photographic film in minutes.25

The concerns for possible health risks related to the residual radioactivity of the irradiated gemstones led to government regulations in many countries.26 In the United States, the Nuclear Regulatory Commission (NRC) has set strict limits on the allowable levels of residual radioactivity before an irradiated gemstone can be distributed in the country.27 All neutron- or electron beam-irradiated gemstones must be tested by an NRC-licensee prior to release for sales; however, when treated in a cobalt-60 gamma ray facility, gemstones do not become radioactive and thus are not under NRC authority.28 In India, the Board of Radiation and Isotope Technology (BRIT), the industrial unit of the Department of Atomic Energy, conducts the process for private sectors.29 In Thailand, the Office of Atoms for Peace (OAP) did the same, irradiating 413 kilograms (911 lb) of gemstones from 1993 to 2003,30 until the Thailand Institute of Nuclear Technology was established in 2006 and housed the Gem Irradiation Center to provide the service.3132

Materials and results

Effects of irradiation onvarious gemstone materials
MaterialStarting colorEnding color
AmberLight yellowOrangey red,33orangey yellow34
BerylColorlessYellow35
BlueGreen36
Colorlessto pale pink(Maxixe-type)Deep blue37
DiamondColorless oryellowto brownGreen to blue38
FluoriteColorlessVarious39
PearlLight colorsBrown,40gray to black41or gray-blue42
QuartzColorless toyellow orpale greenAmethyst,4344brown,45 rose,46"smoky" (light gray)47
SapphirePink withblue tintTint removed48
TopazYellowto orangeIntensify colors49
Colorlesstopale blueBrown,50dark blue,51green,52sky blue53
TourmalineColorlesstopale colorsBrown,54green-red (bicolor),55intense pink,56pink,5758 red,59yellowish orange60
PinkIntense pink,61orangey pink62
BluePurple63
ZirconColorlessBrown to red64

The most commonly irradiated gemstone is topaz, which usually becomes blue after the process.65 Intensely blue topaz does not exist in nature and is caused by artificial irradiation.66 According to the American Gem Trade Association, approximately 30 million carats (6,000 kg or 13,000 lb) of topaz are irradiated every year globally, 40 percent of which were done in the United States as of 1988.67 Dark-blue varieties of topaz, including American Super Blue and London Blue, are caused by neutron bombardment,68 while lighter sky-blue ones are often caused by electron bombardment.69 Swiss Blue, subtly lighter than the US variety, is caused by a combination of the two methods.70

Diamonds are mainly irradiated to become blue-green or green, although other colors are possible.71 When light-to-medium-yellow diamonds are treated with gamma rays they may become green; with a high-energy electron beam, blue.72 The difference in results may be caused by local heating of the stones, which occurs when the latter method is used.73

Colorless beryls, also called goshenite, become pure yellow when irradiated, which are called golden beryl or heliodor.74 Quartz crystals turn "smoky" or light gray upon irradiation if they contain an aluminum impurity, or amethyst if small amounts of iron are present in them; either of the results can be obtained from natural radiation as well.75

Pearls are irradiated to produce gray blue or gray-to-black colors.76 Methods of using a cobalt-60 gamma ray facility to darken white Akoya pearls were patented in the early-1960s.77 But the gamma ray treatment does not alter the color of the pearl's nacre, therefore is not effective if the pearl has a thick or non-transparent nacre.78 Most black pearls available in markets prior to the late-1970s had been either irradiated or dyed.79

Uniformity of coloration

Gemstones that have been subjected to artificial irradiation generally show no visible evidence of the process,80 although some diamonds irradiated in an electron beam may show color concentrations around the culet or along the keel line.81

Color stability

In some cases, the new colors induced by artificial irradiation may fade rapidly when exposed to light or gentle heat, so some laboratories submit them to a "fade test" to determine color stability.82 Sometimes colorless or pink beryls become deep blue upon irradiation, which are called Maxixe-type beryl. However, the color easily fades when exposed to heat or light, so it has no practical jewelry application.83

Notes

Citations

Works cited

References

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  13. Generally speaking, either photons having energy of at least 10 MeV or neutrons are needed to induce significant radioactivity in a material.[8] /wiki/MeV ↩

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  17. As of 1987, most developed countries regarded 2 nanocuries per gram (74 Bq/g) as safe to release to the public while the U.S. federal release limits for most nuclides were 1 nanocurie per gram (37 Bq/g) or less, and that of the United Kingdom was 2.7 nanocuries per gram (100 Bq/g).[11] As of 2022, the release limit of the European Union is 2.7 nanocuries per gram (100 Bq/g).[10] ↩

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