Dual-color double stars in ruby, sapphire, and quartz: Cause and historical account

A largely overlooked form of asterism consisting of dual-color double stars is found in natural sapphire, in diffusion-treated and non-diffusion-treated synthetic rubies and sapphires, and in natural quartz. To characterize and explain this phenomenon, examples of these materials were examined. In transparent or translucent samples, an optical pattern of a white six-rayed star and a bodycolored (e.g., red, orange, yellow, green, blue) six-rayed star was frequently observed. Grinding and repolishing experiments showed that the asterism of part of the synthetic samples was produced or enhanced by diffusion treatment. The mechanism responsible for the formation of the dual-color double stars is discussed. To produce the pattern, acicular inclusions must either be present in relatively thin layers confined to the dome and the base of diffusion-treated ruby or sapphire cabochons, or be distributed throughout the complete corundum (natural or synthetic) or quartz samples. The white star is caused by interaction of light with the upper layer of the cabochon's dome. The bodycolored star, in contrast, is generated by light that enters the cabochon, is reflected and scattered at the base layer of the cabochon, and then travels a second time back through the body of the sample. As further prerequisites for observation of the phenomenon, the gemstones must be transparent or translucent, with polished base and dome. A historical summary of the manufacture and improvement of synthetic asteriated corundum by diffusion treatment offers additional insight into dual-color double-star stones. According to firsthand accounts and patent documents, diffusion has been used to produce or enhance asterism in synthetic and natural corundum since the 1950s. The treated material has been released commercially since at least the 1970s, and it is still produced and found on the international market.