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

The color stability of a large natural yellow sapphire attributed to Sri Lanka was investigated after its owner reported apparent fading following a period on consignment. The rectangular faceted stone weighed 45 ct and measured 20 mm in length. Its physical properties identified it as corundum, while characteristic inclusions and strong orange fluorescence under 365 nm ultraviolet radiation supported the stated geographic origin. Initially, it displayed a saturated yellow color and weak yellow–pale yellow dichroism. A double-beam spectrophotometer provided an objective record before experimentation: transmission was relatively broad and even between 800 and 550 nm, declined steeply below 550 nm, and reached zero at approximately 340 nm. With the owner’s permission, the stone was exposed for 36 hours to a 365 nm ultraviolet lamp to investigate possible bleaching by the ultraviolet component of daylight. This produced a substantial loss of color saturation, establishing instability under the experimental conditions. Subsequent irradiation with X-rays at 60 kV and 25 mA for three hours progressively intensified the color, with visible changes at hourly intervals, ultimately producing a saturated orangy yellow and more distinct dichroism. The resulting transmission curve showed a broad maximum near 675 nm, a stronger decrease below 600 nm, and zero transmission near 420 nm. Microscopic examination indicated that the induced color extended throughout the stone. Heating was not attempted because of the possibility of damage. To demonstrate reversibility, most of the sapphire was shielded with aluminum foil, leaving approximately one-third exposed to the same ultraviolet lamp under similar conditions. The exposed end faded markedly while the protected portion retained its stronger color, producing a conspicuous contrast within the specimen. These experiments confirmed that X-ray irradiation could enhance the color and ultraviolet exposure could reverse that enhancement in this sapphire, consistent with the laboratory’s previous experience with other yellow sapphires. The findings draw attention to the commercial implications of unstable, artificially induced coloration without establishing whether the stone had been treated before submission. The effect is distinguished from superficial color enhancement confined to a thin outer layer, which may be partly removed by recutting.