In which type of stone is creating asterism with heat treatment most commonly applied?

Creating asterism, which refers to the star-like pattern seen on the surface of certain gemstones when they are cut in a cabochon shape, is most commonly associated with corundum. This phenomenon occurs due to the presence of needle-like inclusions, such as rutile, which reflect light in a way that produces a star effect.

In the case of heat treatment applied to corundum, such treatments can enhance the visibility and strength of the asterism, making the star effect more pronounced. While heat treatments are also used for other stones, such as topaz or quartz, they do not typically relate to the creation of asterism in the same way that they do with corundum.

Diamonds, while they can exhibit various optical phenomena, do not typically show asterism in the manner that corundum does. Thus, the most common application of heat treatment to enhance asterism occurs in corundum, specifically in star sapphires and star rubies, where the combination of the right inclusions and heat treatment can bring out the stunning visual effect associated with these gems.