R (Irradiation) — The AGTA Code for Radiation Treatment
R (Irradiation) — The AGTA Code for Radiation Treatment
The standard disclosure designator for gamma, electron-beam, and neutron-bombardment colour treatments
R is the AGTA enhancement code used to indicate that a gemstone has been treated with ionising radiation — gamma rays from a cobalt-60 source, accelerated electrons, or neutron bombardment in a nuclear reactor — to alter or intensify body colour. The code is part of the disclosure framework adopted by the American Gem Trade Association in the early 1990s and now incorporated into Confédération Internationale de la Bijouterie standards as the international reference for treatment terminology.
Materials commonly irradiated
The largest commercial application of irradiation is the production of blue topaz from colourless rough. Untreated colourless topaz is heated, bombarded in a nuclear reactor with neutrons (producing the saturated blue colours marketed as London Blue) or in a linear accelerator with electrons (producing the lighter Sky Blue), and then heated again to remove residual brown components. Without irradiation, blue topaz of saturated colour and large size would be commercially negligible.
Other commonly irradiated materials include diamond (producing fancy green, blue, black, and pink colours, often combined with subsequent annealing), smoky and prasiolite-green quartz, certain pearls (producing dark grey and black colours), and some tourmaline. Beryl, kunzite, and certain sapphires also respond to irradiation, though the colours produced may be unstable to light or heat in some cases.
Stability and disclosure
Most irradiation treatments produce stable colour under normal wear and storage conditions. Blue topaz is colourfast and durable, as is irradiated black diamond. Some irradiation-induced colours, however, fade with prolonged exposure to light or heat — irradiated yellow and brown sapphire, certain irradiated kunzite, and some treated tourmaline are notable examples. Disclosure obligations under AGTA standards apply regardless of stability.
Irradiated gemstones must clear regulatory residual-radioactivity limits before commercial release. In the United States, the Nuclear Regulatory Commission requires that irradiated topaz be held in licensed cooling facilities until residual activity falls below specified thresholds — a process that takes weeks to months for reactor-treated material. Compliance is documented and enforced; non-compliant material is illegal to sell.
Combined treatments
Irradiation is often combined with subsequent heat treatment — particularly for blue topaz, where the post-irradiation annealing removes brown defect colours and stabilises the blue. Combined treatments are coded R + H or in the longer descriptive form (irradiated and heated). Trade practice requires that all relevant treatments be disclosed; partial disclosure (declaring only one component of a combined treatment) is non-compliant. The major laboratories — GIA, AGL, Gübelin, SSEF — identify irradiation through a combination of spectroscopic analysis and inclusion examination, and routinely note treatments on their reports.
Identification
Irradiation is identifiable in the laboratory primarily through ultraviolet-visible-near-infrared spectroscopy. Irradiation produces characteristic absorption bands and colour-centre peaks that distinguish treated from naturally coloured material. In diamond, irradiation often introduces the GR1 colour centre (a vacancy defect with absorption at 741 nm); in topaz, the colour-centre signature differs between gamma-, electron-, and reactor-treated material. Inclusion examination contributes secondary evidence — naturally coloured diamonds and quartz often show colour zoning aligned with growth zones, while irradiated material tends to show colour distributed uniformly through the body.