31.08.2022 Irradiation of Gemstones
Irradiation is a rarely used method of treating gemstones by which the color properties of a gemstone can be altered. In addition to artificial irradiation as a treatment method, there is naturally occurring radioactivity in minerals and gemstones such as amethysts that owe their color intensity to natural radiation.
How does artificial irradiation of gemstones work, for which gemstones is it used and are they radioactive as a result?
Natural radiation of gemstones vs. irradiation as treatment
In the research of the causes of color of gemstones, it was found out at the beginning of the 20th century that, for example, amethysts, yellow sapphires and green diamonds owe their color intensity to natural radiation. The source of radiation can be the surrounding rock or, in rarer cases, cosmic radiation. Even if the coloration is due to radiation, the minerals themselves no longer emit radiation.
Some minerals like ekanite, heliodor, monazite and zircon contain radioactive elements – thorium or uranium – which are responsible for them to show weak radioactive radiation. In the case of zircon, there can be so many radioactive inclusions that this even destroys the crystal structure.
Since about the middle of the 20th century, gemstones have also been deliberately irradiated to change their color. It was started with smoky quartz, but the method is most often used with topazes.
How gemstones are irradiated
Mostly electron or gamma rays are used, this way there is no radioactivity in the stones. If gemstones are irradiated with neutrons, radioactive substances are produced in small amounts. However, the radiation intensity decreases again within a short time. While electron and gamma radiation are available in laboratories, neutron radiation is only available in nuclear reactors, which means that the effort and cost are much higher.
Which gemstones are irradiated?
Topazes are the most commonly irradiated stones. Gamma, neutron or electron beams are used to enhance their color. The most famous color produced by radiation is “london blue”, a bright petrol blue. A bright light blue (“swiss blue”) and aquamarine blue (“sky blue”) can also be produced in topaz by irradiation.
Diamonds are bombarded with neutrons in nuclear reactors; these color the stones green to deep blue and penetrate deep into the stone. Irradiation with protons, deuterons or alpha particles also produce a green color, but this does not have a great penetration depth. A treatment with accelerated electrons produces blue to greenish-blue colors.
The effort of irradiation must of course be in relation to the expected gain and the color improvement obtained must also be stable. For example, dark blue beryls and yellow sapphires produced from colorless stones by irradiation fade in sunlight, as do green or yellow spodumene produced from pink spodumene. The effort is essentially profitable only for blue topaz, colored diamonds and red tourmaline.
Topazes are the most commonly irradiated stones. Gamma, neutron or electron beams are used to enhance their color.
Are irradiated gemstones radioactive?
In any case, irradiation treatment must be declared in the trade for gemstones. However, wearing and storing irradiated gemstones is not dangerous, according to the US NRC (Nuclear Regulatory Commission). It is important to pay attention to the decay time of neutron-irradiated gemstones, i.e. the time until the radiation exposure has fallen below specified limits (in the EU this is 100 Bq per gram). Until this is the case, several years usually pass after the treatment – only then are the gemstones allowed to enter the market.