Surface cobalt-diffused sapphire identification: absorption spectra and immersion tests?
7 Answers from travellers
Cobalt diffusion treatment is readily identified using optical spectroscopy and immersion microscopy. Under a handheld spectroscope, cobalt-treated stones exhibit three diagnostic absorption bands in the orange-red spectrum (between 530nm and 620nm), contrasting with natural blue sapphires whose color is produced by iron-titanium intervalence charge transfers showing a 450nm line. Under methylene iodide immersion, cobalt diffusion reveals patchy, surface-conformal color concentration hugging facet junctions, contrasting with untreated stones that display natural hexagonal zoning aligned with the crystal face.
Cobalt-diffused sapphires frequently turn bright pinkish-red when viewed under a Chelsea Color Filter, whereas natural iron-rich Sri Lankan blue sapphires appear inert dark green or grey.
Exporting treated stones mislabelled as "natural unheated corundum" on commercial export invoices violates the National Gem and Jewellery Authority export assay declarations.
Dealers in Beruwala refer to synthetic and treated stones as "cook-stones"; demand written disclosure stating "Natural Corundum, No Indications of Treatment".
Be aware of re-cut stones; lapidaries occasionally repolish tables, producing pale patches where thin cobalt diffusion layers have been cut away.
Use high-CRI halogen illumination during immersion; modern cool white LED desk lamps wash out subtle color concentration rims.
Rely on independent laboratory testing; roadside loupe inspections are unreliable for identifying microscopic diffusion rims.
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