corundum, treatments.6, gems

The visual identification of heat treatment in ruby and sapphire is reviewed through changes in ultraviolet fluorescence and the microscopic appearance of inclusions, illustrated by eight photomicrographs. Heating is used to improve clarity by dissolving rutile silk, modify or enhance colour, and sometimes develop asterism in titanium-rich material. Because treatment produces no appreciable changes in refractive index, birefringence, specific gravity, dichroism, or visible absorption spectra, examination of internal features provides the principal means of recognition. Ultraviolet responses offer supplementary evidence. The apricot fluorescence commonly observed in untreated yellow-orange, yellow, and colourless Sri Lankan sapphires is reduced or eliminated by heating. Some heated blue sapphires display patchy, chalky green fluorescence under short-wave ultraviolet light, potentially resembling Verneuil synthetic blue sapphire. In heated natural stones, however, this fluorescence is confined to colourless areas, while blue zones remain inert; the synthetic material is described as fluorescing evenly across its surface. Heated rubies may exhibit unusual patches of chalky white short-wave fluorescence in addition to their normal red response. Microscopic indicators include partial dissolution of rutile needles, thermal alteration of included crystals, and modification of secondary fluid inclusions. Long, continuous rutile needles oriented in the characteristic directions of corundum are presented as evidence of an untreated stone, although remnants of silk may survive heating. Unequal thermal expansion can produce roughly circular, glassy tension fractures around included crystals, sometimes with rims indicating incipient healing. Crystals may become white, rounded “snowballs”, develop surrounding zones of melted material, or form negative crystals containing colourless or brown glass. In rubies, these altered inclusions may occur within partially healed fractures. Heating can also remove reddish orange staining from secondary fluid inclusions and transform their tubular liquid channels into strings of droplets that follow the original pathways. Surface diffusion treatment is considered separately because its colour is concentrated in a shallow surface layer that may be removed by repolishing. Diagnostic features include stronger colour at the girdle and facet edges, unequal colour concentration between facets, and colour extending into surface-reaching fractures or around inclusions connected to them. These features are most readily examined by immersion in methylene iodide under a microscope with diffused illumination; a dark-field arrangement fitted with a diffuser can reveal many of the same characteristics when immersion is unavailable. The article emphasises practical recognition using ordinary gemmological magnification, ultraviolet examination, and immersion techniques.