Yellow Sapphire

(via ICA Lab Alert, No.9, September 1, 1987) Kurt Nassau writes:

Background
There are several types of natural yellow sapphires that are seen in the trade, including the untreated, the high temperature heated, and the irradiated ones. The first two are stable to light, while the third (irradiated either by nature or by man) fades in light. Natural yellow stones after being mined may fade on light exposure, and it is customary to expose such material to light or heat it. A heating test is also sometimes used to check yellow sapphire for fading: Webster recommends 230°C (446°F) for a few minutes and Nassau has used 200°C for one hour to establish a potential for fading in light in irradiated gemstones in general.

Observation
Ordinary yellow sapphire, that is the non-irradiated, non-heated, non-light fading, stable material can lose some color at as low as 60°C (140°F), more at higher temperatures, and all color by 600°C (1112°F). Quite unexpectedly, light has been found to reverse this change. It restores this type of yellow sapphire to its ‘proper’ stable color from either the dark irradiated state or from the lighter heated state. If heating has been performed accidentally, the color may be restored by exposure to bright light for a few days.

Recommendations
Do not use a heating test for any yellow sapphire. To test for irradiated stones, a light exposure test is the only one that can safely be recommended.

Reference
A fully detailed article by Kurt Nassau and G Kay Valente has been submitted for publication in Gems & Gemology under the title “The Seven Types of Yellow Sapphire and a Corundum Conurundum.”