corundum, sapphire, Sri Lanka.12, treatments.6, gems

Examines the commercial heat treatment of Sri Lankan geuda corundum and considers the origin of its colour response and possible criteria for recognising treated stones. Gunaratne places this development within the established practices of heating ruby to reduce purplish tones and heating dark or unevenly coloured sapphire to improve appearance. Geuda is described as a milky, opalescent condition associated with abundant inclusions, particularly rutile, which had formerly reduced the commercial value of the rough. Its successful conversion into blue sapphire transformed demand, initially through extensive treatment in Thailand and subsequently through growing activity in Sri Lanka. Rough selection is presented as crucial: favoured material combines a faint powder-blue tint, milky sheen and honey-like transmitted appearance, locally termed diesel geuda. Fractures, fluid inclusions and other potentially damaging features influence suitability, while different grades require different treatment conditions. Gas-, diesel- and electrically heated furnaces are described as producing comparable transformations, with heating duration adjusted to the material and temperatures achieved. The author proposes that differences in the origin and crystallisation history of rutile help explain variable treatment responses, linking intense heating to redistribution of titanium-related colouring constituents within the host. These explanations are offered as theoretical interpretations requiring further investigation rather than results of controlled chemical experiments. Examination of successfully treated stones reveals diffuse or blotchy colour zoning, reduced sharpness of growth lines, occasional woolly white patches and healed fractures. Zircon inclusions and fissure stains were absent from the examined material, although their absence could reflect selection of the original rough. Most specimens were inert under short-wave ultraviolet radiation; only four deep-blue polished stones in a selected group of one hundred rough and polished samples showed a powder-blue glow. Some stones exhibited dirty brown to brownish-red luminescence under long-wave ultraviolet radiation. Optical and physical constants and handheld absorption spectra remained comparable with untreated sapphire, limiting their diagnostic usefulness. The paper calls for nondestructive identification methods, systematic comparison of stones before and after heating, and investigation of the critical temperatures governing colour development. Its conclusions are explicitly limited by reliance on commercially treated, successful products and insufficient examination of untreated starting material or failed treatment outcomes.