synthetic corundum.7, gems

This three-part series reviews methods available in 1946–1947 for distinguishing synthetic corundum, spinel, and emerald from their natural counterparts, emphasizing microscopic examination of inclusions and growth structures. R.T. Liddicoat introduces the distinction between a synthetic gem, which reproduces the chemical composition and atomic structure of its natural counterpart, and an imitation marketed under an inaccurate name. Materials sold as synthetic alexandrite, zircon, garnet, or aquamarine are described as corundum or spinel substitutes identifiable by their different properties. For natural and synthetic corundum, however, ordinary refractive-index and density measurements provide little assistance, making internal features particularly important. Natural ruby and sapphire are illustrated by fingerprintlike fluid inclusions, zircon crystals with surrounding fractures, oriented silk intersecting at 60°, and straight or hexagonally arranged growth zoning. Synthetic material produced by the Verneuil process is characterized by spherical or elongated gas bubbles and curved growth striae. The authors explain that nearly flawless specimens require higher magnification, careful control of illumination, examination from different directions, and sometimes immersion to reduce distracting surface reflections. E.J. Gübelin expands the discussion of gas inclusions, describing isolated bubbles, localized clouds, and concentrations following growth layers, while noting that improvements in production reduce their abundance and size. He cautions that subsurface cracks caused by rapid polishing occur in both natural and synthetic corundum and therefore do not establish synthetic origin. The third installment treats synthetic spinel separately because curved striae are absent from the material examined. Its gas inclusions may be spherical, tubular, wormlike, or irregularly elongated, while characteristic strain patterns under crossed polarizers provide further evidence. Gübelin contrasts the cloudy, fibrous, or feathery anomalous birefringence of synthetic spinel with the generally fainter, less sharply defined patterns observed in natural specimens. Synthetic emerald presents a different problem because its slower crystallization can produce straight or hexagonal zoning resembling natural growth. Slightly lower refractive index, density, and birefringence are discussed as supporting distinctions, but microscopic features receive greater practical emphasis. These include scattered minute solid particles, undulating wisps or feathers described as groups of small liquid inclusions containing gas bubbles, irregular cavities, and pronounced strain-related anomalous birefringence. Gübelin treats the identity of the scattered solid particles as uncertain, tentatively suggesting coloring material. Recommended observation methods include dark-field microscopy, transmitted light, crossed polarizers, and immersion, with the lighting selected to reveal particular structures. Spectroscopy and luminescence are mentioned as supplementary approaches when microscopy proves insufficient. Throughout, the series presents identification as a matter of recognizing diagnostic internal features while avoiding conclusions based on ambiguous cracks, superficial resemblance, or the absence of conspicuous imperfections.