corundum, color.4, synthetic corundum.7, gems

E. Burbage and T.G. Jones present preliminary observations of colour and optical changes produced by neutron irradiation in a range of natural and synthetic gemstones. Pairs of specimens were assembled for comparative study, with one member irradiated for three or six days and the other retained untreated. The resulting collection was exhibited at Harwell and subsequently at Haslemere’s Educational Museum. Synthetic corundum showed particularly conspicuous and variable responses. A colourless boule became brown, developed strong dichroism, and displayed enhanced fluorescence under crossed filters, which the authors tentatively associate with trace chromium impurities. Synthetic ruby darkened and became more strongly dichroic while showing reduced fluorescence; pink synthetic sapphire turned brown and likewise fluoresced less strongly. Two blue synthetic sapphires exposed for six days responded differently: one became dark brown with little appreciable dichroism, whereas the other became greenish blue with pronounced directional colour differences. An alexandrite-type synthetic corundum became garnet red and lost its change of colour between different illuminants. Natural corundum changed less markedly, with no appreciable alteration in the ruby examined and only slight darkening in sapphire. Synthetic spinels showed either substantial fading towards pale grayish brown or a shift towards a steely blue. Among quartz varieties, cairngorm darkened slightly, while amethyst and rose quartz showed stronger darkening. Pink topaz returned to a sherry-brown appearance resembling its colour before heating, and pale-yellow topaz became darker and greener with increased dichroism. Blue and golden zircons underwent especially dramatic changes after only three days, becoming deep reddish brown and nearly opaque, although examination at their edges confirmed that they remained anisotropic. Garnets were comparatively little affected: rhodolite appeared unchanged, almandine deepened slightly, and demantoid shifted towards a peridot-like green. Additional changes included greener brown tourmaline, sherry-brown kunzite, brownish-green peridot, deeper golden beryl, and brown orthoclase. Colourless periclase developed a blue resembling chalcedony, judged the most attractive result. The observations demonstrate that irradiation can alter dichroism and fluorescence as well as visible colour, with markedly different responses even among nominally similar materials. The authors frame the work as an interim report rather than a comprehensive survey of gemstone irradiation.