corundum, synthetic corundum.7, gems

A 2.35 ct faceted sapphire is described in which conspicuous polysynthetic twin lamellae closely resembled those commonly associated with natural corundum, although further examination established a Verneuil synthetic origin. The stone’s unusually intense colour initially raised suspicion of treatment, but colour alone was considered unreliable for distinguishing natural from synthetic sapphire. Examination with a 10× lens revealed a few straight, parallel twinning lines in an otherwise particularly clean stone. The girdle lacked the pockmarked appearance sometimes associated with heating, while immersion in methylene iodide suggested that colour was concentrated near the surface. The iron absorption line near 450 nm was absent, and the stone remained inert under both long-wave and short-wave ultraviolet radiation. Immersion microscopy between crossed polarisers readily revealed the straight twin lamellae, including interference rings along the twin planes. Because such twinning can occur exceptionally in Verneuil corundum, its presence was not accepted as conclusive evidence of natural origin. Examination along the optic axis under crossed polarisers produced the characteristic Plato striation pattern, described as a lozenge-shaped arrangement formed by intersecting sets of lamellae. Curved colour bands, further supporting the Verneuil identification, were difficult to observe and became visible only during immersion with illumination diffused through tissue paper. The author distinguishes the conspicuous polysynthetic lamellae from the structures responsible for the Plato pattern because they followed different directions. The former appeared to lie in planes slightly inclined to the optic axis, but their precise crystallographic orientation could not be established from the faceted specimen. Although all Verneuil synthetics previously examined by the author had yielded a positive Plato test, this was the only sapphire encountered with such convincing natural-like polysynthetic twinning. The report therefore stresses the importance of seeking curved colour banding or Plato striations before identifying a sapphire from twinning alone. Possible surface diffusion treatment is also discussed because of the apparent surface concentration of colour and the absence of the usual chalky bluish green short-wave fluorescence. This interpretation remained tentative, since the stone could not be tested by repolishing. The case demonstrates how an unusual combination of internal structures and possible treatment effects can complicate sapphire identification.