Multi-step irradiation is a sequential treatment protocol applied to diamonds to produce fancy colour through a combination of irradiation and annealing (controlled heating) steps. The treatment sequence creates colour centres in the diamond lattice through irradiation, then modifies or stabilises those centres through subsequent annealing at carefully controlled temperatures. The process is most commonly used to produce fancy yellow, orange, and green colours in diamonds that originally were near-colourless or off-white, with the specific colour outcome depending on the original diamond's nitrogen content and structural features and on the specific sequence and conditions of the treatment steps. Detection of multi-step irradiation requires advanced laboratory analysis, with all major diamond grading laboratories disclosing the treatment on their reports under FTC and equivalent international disclosure requirements.

The treatment sequence

The basic multi-step irradiation sequence involves two principal steps. The first is irradiation, typically using either electrons (in a particle accelerator) or gamma rays (from a cobalt-60 source), at energies and doses sufficient to displace carbon atoms from their lattice positions and create the vacancy and interstitial defects that function as colour centres. Electron irradiation is more commonly used in commercial practice because of its more controlled and predictable outcomes; gamma irradiation has historically also been used but with less precision in dose control.

The initial irradiation typically produces a green colour through the creation of GR1 colour centres (single neutral vacancies in the diamond lattice). The green colour is intense and visually striking but is generally not the desired commercial outcome, as the green colour is unstable to further heating and will modify with subsequent annealing.

The second step is annealing — controlled heating of the irradiated diamond at temperatures typically in the range of 500 to 800 degrees Celsius. The annealing causes the GR1 centres to migrate and combine with nitrogen impurities (which are present at varying concentrations in most natural diamonds) to form different colour centres. The N-V (nitrogen-vacancy) and H3 centres produced through annealing typically appear yellow or orange-yellow, with the exact colour depending on the specific centre populations and the nitrogen aggregation state of the original diamond.

Variations and refinements

Beyond the basic two-step protocol, more elaborate sequences are used in commercial practice for specific colour outcomes. Some yellow and orange diamonds are produced through additional annealing steps at successively higher temperatures, with each step shifting the colour balance toward the desired final outcome. Some treatments combine multi-step irradiation with HPHT (high-pressure high-temperature) treatment, which can substantially modify the underlying nitrogen aggregation state of the diamond and thereby change the colour centres available for the subsequent irradiation steps. The combination of HPHT and multi-step irradiation can produce colour outcomes (including some pink and blue colours) that are not achievable through irradiation alone.

The specific protocols used by commercial treatment houses are often closely held, with the precise temperature, dose, and timing parameters representing the commercial expertise of the firm. Some treatment houses are known for particular colour specialisations, with the resulting treated diamonds carrying recognisable colour signatures that experienced graders can sometimes associate with particular treatment sources.

Detection and disclosure

Multi-step irradiation is detectable by laboratory analysis through several characteristic signatures. The colour centres produced by the treatment have specific spectroscopic features that distinguish them from the colour centres of natural-colour diamonds. Photoluminescence imaging at low temperature (typically with the diamond cooled in liquid nitrogen) reveals the spatial distribution of the colour centres in ways that often differentiate treated from natural colours. The combination of these techniques, supplemented by careful microscopic examination of growth features and any inclusion patterns, supports confident identification of multi-step irradiation by major laboratories.

All major diamond grading laboratories — GIA, AGS, IGI, HRD — disclose multi-step irradiation and the broader category of irradiation treatment on their grading reports. The disclosure is mandatory under the US Federal Trade Commission Jewelry Guides and equivalent international disclosure requirements, with treated-colour diamonds required to be sold with clear identification of the treatment status.

Market acceptance and pricing

Multi-step irradiated diamonds occupy a specific commercial niche in the broader fancy-colour diamond market. The treatment provides access to colours (vivid yellows, oranges, greens) that would otherwise require natural-colour diamonds at substantial premiums. Treated-colour diamonds typically sell at significant discounts to natural-colour equivalents — often 10 to 30 per cent of the natural-colour pricing for comparable quality, depending on the colour and the specifics of the market segment. The discount reflects both the reduced rarity of treated material and the buyer preference for natural colour wherever the budget supports it.

The treated-colour diamond market has matured substantially over the past several decades, with mature commercial protocols, reliable laboratory documentation, and established price relationships supporting a stable and well-functioning market. Treated diamonds are widely used in commercial fancy-colour jewellery, particularly in earrings and pendants where the colour effect is visible and attractive but the absolute investment value of natural-colour material is not required.

In the trade

For Skyjems and the broader trade, multi-step irradiated diamonds represent an important commercial category that supports access to fancy colours at price points well below natural-colour equivalents. Full disclosure of the treatment status is essential under both regulatory requirements and trade ethics, with the laboratory documentation providing the supporting evidence. The category is particularly relevant for commercial jewellery applications where the colour effect is the primary value driver and the natural versus treated origin is less critical than the visible result. Buyers seeking the highest investment value will continue to prioritise natural-colour material with appropriate laboratory documentation.

Further reading

• GIA — Treatment research and detection
• GIA — Fancy Color Diamonds research
• International Gem Society — Diamond treatments