What effect does heat treatment in a reducing environment have on sapphire?

Heat treatment in a reducing environment specifically enhances the blue color of sapphire. This process involves heating the stone in environments with limited oxygen, which alters the chemical structure of the trace elements within the sapphire. In particular, the presence of iron and titanium can lead to strong blue color saturation when subjected to high temperatures.

During this heat treatment, the removal or alteration of color-causing impurities intensifies the blue hue, resulting in a gem with a more vibrant and deeper coloration. This technique is commonly used in the gemstone industry to improve the overall appearance of sapphires, making them more appealing to consumers.

The process does not typically lighten the color or directly enhance clarity, nor does it create asterism, which is a unique optical phenomenon caused by the presence of needle-like inclusions rather than by heat treatment.