An irradiated diamond is a natural or laboratory-grown diamond whose colour has been altered by exposure to high-energy particles or electromagnetic radiation. The treatment can produce green, blue, yellow, orange, brown, pink, and black diamonds, depending on the starting material and on whether the irradiation is followed by annealing. Diamond irradiation is one of the oldest documented gem treatments, with the first experiments by Sir William Crookes in 1904 producing green colour in colourless diamonds exposed to radium salts. Modern irradiation has been a routine industrial process since the 1950s.

The mechanism of colour change

Irradiation produces colour by displacing carbon atoms from their lattice positions, creating vacancies, interstitials, and complex defects that absorb light at particular wavelengths. The most common defect produced by neutron or electron irradiation in colourless diamond is the GR1 centre, a single neutral vacancy that absorbs in the red and yields a green body colour. Subsequent annealing at around 800 degrees Celsius converts vacancies, often paired with naturally occurring nitrogen, into colour centres such as the H3 and H4 centres (yellow), the N-V centre (pink to red), and the 595-nanometre centre. The progression of colour with controlled annealing is well documented and underlies the production of fancy yellow, pink, and orange irradiated diamonds.

Treatment routes

Three principal irradiation routes are used. Cyclotron and linear accelerator electron irradiation produces shallow surface colour, typically green, restricted to a thin shell around the stone. Gamma irradiation from cobalt-60 sources produces deeper colour but with limited intensity. Neutron irradiation in nuclear reactors produces the deepest and most uniform colour change, often body-coloured throughout the stone. The choice of route depends on the depth and intensity of colour required and on the diamond's nitrogen content.

After irradiation, diamonds are typically annealed at temperatures from 600 to 1,000 degrees Celsius to develop the target colour. The combined irradiation-and-annealing sequence is the source of nearly all commercial fancy-coloured treated diamonds.

Detection

The major laboratories, GIA, IGI, HRD, and the Swiss Gemmological Institute, identify treated colour in diamonds through low-temperature ultraviolet-visible-near-infrared spectroscopy and photoluminescence. Each colour centre has a diagnostic absorption or photoluminescence signature: the GR1 centre at 741 nanometres for green, the H3 at 503 nanometres and H4 at 496 nanometres for yellow, and the 595-nanometre centre for various colours. Combinations of these features distinguish irradiated stones from naturally coloured diamonds, where the colour-centre concentrations follow different statistical patterns.

Irradiated stones receive coloured-diamond reports clearly stating that the colour origin is treated, with the treatment described as irradiation, often with annealing. This disclosure is required by CIBJO, the FTC, and laboratory reporting standards.

Pricing and market

Irradiated coloured diamonds trade at a substantial discount to natural-colour stones of equivalent appearance. A treated fancy vivid yellow may sell at ten to twenty per cent of the price of a comparable natural fancy vivid yellow, and treated fancy blue or pink stones at even lower fractions of natural price. The market for treated coloured diamonds is genuine and accepted, but turns on full disclosure and laboratory documentation.

Stability

The colour produced by irradiation and annealing is generally stable under normal wear and jewellery cleaning. High-temperature heat exposure such as a jeweller's torch can reverse some colour centres, and in particular the H3 yellow can fade if the diamond is heated unprotected. Routine bench work at modest temperatures with the stone protected does not affect the colour.