London Blue is a trade name for a deep grey-blue colour grade of topaz produced by the irradiation and subsequent heat-treatment of natural colourless topaz. It is one of three principal blue topaz colour grades in current commercial use, alongside Sky Blue (the lightest) and Swiss Blue (the brightest medium tone). The London Blue grade has the deepest and most saturated colour of the three, with a characteristic slightly greyish or slightly inky appearance that is easily distinguished from the brighter Swiss Blue and the paler Sky Blue.

Production

The London Blue colour is produced by exposing colourless topaz to high-energy radiation, typically high-energy electron-beam irradiation in a linear accelerator, followed by controlled heat treatment to anneal the colour. The treated stones must then be held under controlled conditions until the radioactive isotopes induced by the treatment decay to safe levels, a process that can take from several weeks to several months depending on the energy of the radiation used. The treated material must meet strict residual-radioactivity standards before being released for trade; in the United States the Nuclear Regulatory Commission and the equivalent bodies elsewhere supervise the standards.

Visual character

London Blue topaz is recognisable by its deep saturated greyish-blue colour, with a tonal range from medium to fairly dark. The body colour is even and consistent across the stone in well-cut goods, and the high refractive index and good clarity that natural topaz typically displays give the cut stone a strong visual presence at modest cost. The stone takes a clean polish, holds an excellent cut, and is durable for normal jewellery wear with reasonable care.

Disclosure

The treatment is permanent and stable under all normal conditions of wear, but it is treatment, and disclosure is required under all major trade disclosure standards including AGTA, CIBJO and the FTC Jewelry Guides. Sellers offering blue topaz as natural without disclosure of irradiation are misrepresenting the goods. Natural blue topaz is a real material but is rare and pale; the deep blue colours of the trade goods are essentially all treated.

Trade significance

London Blue topaz, with its sister grades Sky Blue and Swiss Blue, has been the principal affordable blue-stone in the trade since the irradiation treatment process became commercially established in the 1970s and 1980s. Per-carat prices are low compared with sapphire or aquamarine of similar visual impact, the supply is abundant, and the material is available in large clean stones at sizes that would be very expensive in untreated blue stones of the same visual character. The category has been a workhorse of the mid-market jewellery industry for nearly fifty years, and the London Blue grade in particular is widely used in commercial silver and gold jewellery where a strong saturated blue is required at an accessible price.