What Does It Mean When A Stone Is Heated?
The oldest and most common sapphire enhancement is heat treatment, the majority of the sapphires in the marketplace have been heat-treated or thermally enhanced in furnaces. Heat treatment is used to improve a sapphire’s color, remove color zoning, and improve clarity.
Some heat treatment procedures are relatively mild, involving temperatures of about 400o C for brief periods of time. These are commonly used on pink sapphires from the mines of Madagascar and may also be applied to Madagascar’s coveted padparadscha sapphires.
Over the last couple of decades, high temperatures in excess of 1700o C has been applied to sapphires, which improve their clarity by dissolving silk as it improves color. The results can be dramatic, transforming pale stones into ones with lively, saturated colors. Knowledgeable gemologists will be able to discern whether a sapphire has been subjected to this kind of extreme heating.
When corundum is heated to between 1200o-1600oC, star sapphires can be created or improved, however, since the demand for transparent rough has increased in recent decades, treatments to create asterism are not frequently encountered.
While some consumers have learned to accept heat-treated stones, even without disclosure, connoisseurs prize those rare stones that show fine color and clarity without this enhancement. Another common treatment in colored stones is Fracture Filling & Flux Healing.