Almost all blue topaz on the market today is the product of a two-step treatment in which colourless natural topaz is first exposed to ionising radiation and then heated to drive off undesirable colour components, leaving a stable blue. The treatment was developed commercially in the late 1970s and early 1980s and rapidly transformed topaz from a niche golden gemstone into one of the most widely available blue gems in the world.

The problem of natural blue topaz

Natural blue topaz exists, but in colour intensities ranging from very pale to barely perceptible. Localities such as Ouro Preto in Brazil, the Texas Llano uplift, and several Russian sites produce small quantities of naturally blue material, but the commercial supply has always been a tiny fraction of demand. The treatment process was pursued precisely because abundant colourless topaz was available from Brazil, Nigeria, and Sri Lanka, and the goal was to convert it into the blue colour for which the market would pay.

The treatment process

Two principal treatment routes are used. In gamma irradiation, parcels of colourless topaz are exposed in commercial facilities to cobalt-60 gamma sources, producing a pale to medium blue typically marketed as Sky Blue. In linear-accelerator electron-beam treatment, the same starting material is bombarded with high-energy electrons, producing a darker greenish-blue that, after subsequent heat treatment around 200 to 250 degrees Celsius, yields the saturated blue marketed as Swiss Blue. The deepest hue, marketed as London Blue, is produced by reactor neutron irradiation, in which topaz is exposed within a nuclear reactor to a mixed neutron and gamma flux, and then heated. The neutron treatment penetrates deeply and produces an inky greenish-blue that has dominated the deep-blue end of the trade since the 1980s.

Reactor-irradiated topaz must be quarantined after treatment to allow induced radioactivity from trace impurities to decay below regulatory release limits, typically a period of months. The Nuclear Regulatory Commission in the United States and equivalent agencies in other countries require batch testing before stones are released into the market, and treated topaz is not released for sale until residual activity is below the relevant exemption limit. Properly handled treated topaz on the consumer market poses no radiation hazard, a point that has been confirmed in repeated public health assessments.

Stability and disclosure

The blue colour produced by irradiation and heat is stable to normal wear and to ordinary jewellery cleaning, although prolonged exposure to direct sunlight or to heat above approximately 200 degrees Celsius can fade the colour, particularly in lighter-coloured Sky Blue material. The treatment is permanent for practical purposes but is disclosable under CIBJO and FTC rules, and reputable retailers describe the stones as treated or irradiated.

Pricing of irradiated blue topaz is among the lowest of any common gemstone, reflecting the abundance of starting material and the established treatment infrastructure. London Blue at ten dollars per carat in calibrated commercial sizes is unremarkable, and even fine large stones rarely command the prices of comparable natural-coloured aquamarine or sapphire. Natural-colour blue topaz from documented localities, by contrast, can command a substantial premium when accompanied by laboratory disclosure that the colour is untreated.