corundum, ruby, synthetic ruby, synthetic corundum.7, inclusions.5, gems

Gübelin examines the recognition of the newer Chatham, Kashan, and Knischka synthetic rubies, whose inclusions can closely resemble those of natural stones and invalidate several previously accepted diagnostic rules. He notes that color, dichroism, ultraviolet luminescence, and the ordinary absorption spectrum are generally of limited value, although dark Kashan rubies may show a conspicuous orange extraordinary ray and spectrophotometry reveals useful differences in ultraviolet transmission. The principal emphasis is on microscopic inclusions: flux residues may occur as drops, grains, tubes, feathers, veils, nets, wisps, clouds, and lace-like structures, with some Kashan flux inclusions identified by X-ray analysis as cryolite (Na₃AlF₆). Particularly diagnostic Kashan features include “fog,” “hairpins,” and “comets,” while Chatham and Knischka stones may contain triangular or hexagonal platinum platelets with metallic lustre. Needles resembling natural rutile and straight or angular growth bands resembling those of natural ruby demonstrate that neither such needles nor angular zoning can any longer be regarded by themselves as proof of natural origin. Knischka rubies additionally show characteristic negative crystals, flux residues, and distinctive two- and occasionally three-phase inclusions. Numerous photomicrographs illustrate how closely these synthetic inclusion scenes can imitate healed fractures, rutile needles, and growth structures in natural Burmese, Sri Lankan, and Thai rubies. Gübelin concludes that well-equipped laboratories can reliably identify the new synthetics using microscopy together with spectrophotometric and X-ray methods, while jewelers and dealers without such equipment must depend particularly on careful examination of diagnostic inclusions and comparison with known synthetic specimens.