corundum, synthetic corundum.7, assembled stones, gems

F.H. Pough examines newly marketed assembled star stones, comparing their construction, appearance, identification, and durability with earlier attempts to imitate natural star rubies and sapphires. He traces the development from fixed stars engraved on the backs of glass or synthetic gemstones to more convincing moving-star effects obtained from coated star quartz, scratched synthetic corundum, and phlogopite-backed composites. These predecessors demonstrate that movement of a star with viewing angle cannot by itself establish natural origin. Earlier products often suffered from weak optical effects, excessively deep cabochons, unstable coatings, or adhesives that separated during wear. The introduction of epoxy resins revived interest in assembled stones by providing stronger, water-resistant bonds. Pough describes two contemporary constructions employing shallow cabochon tops, principally synthetic corundum or spinel, although glass is also used. In the first, a transparent top is cemented to thin glass backing apparently scratched in one direction for a cat’s-eye or three directions for a star, then coated with a reflective metallic film. The shallow top improves light return, producing a brighter effect than earlier high-domed versions, although the reflected star is more strongly coloured than those of natural or Linde synthetic star stones. The second construction joins a synthetic ruby or sapphire top to an opaque, deeply coloured sintered-corundum backing with a metallic solder layer. Pough infers that microscopic grooves on the underside of the top generate the star, while the solder both bonds the components and supplies the reflecting surface. X-ray examination supports identification of the tested red backing as corundum. This construction produces particularly sharp, bright stars and appears somewhat more resistant to extreme conditions than the epoxy-bonded glass-backed version. Both products nevertheless separate or suffer damage under sufficiently severe heating, although Pough considers ordinary wear unlikely to reproduce those conditions. Recognition is straightforward when the stones are loose, and the unusually dark sintered backing remains a useful clue in mounted examples. He distinguishes three possible locations for the star-producing structures: within the gemstone, on its underside, or in an attached backing. The new products receive qualified approval for beauty and apparent durability, but Pough predicts that their ease of manufacture and competition from poorer glass versions may undermine their commercial appeal.