What does a gem undergo during heat treatment?

During heat treatment, a gem undergoes temperature exposure, which is a critical process used to enhance its color, clarity, or overall appearance. In this process, the gem is subjected to high temperatures, allowing specific alterations to occur in its atomic structure. This can lead to improved color saturation or the removal of undesirable inclusions and color zoning.

For example, sapphires and rubies often experience heat treatment to enhance their vibrant hues. The controlled temperature environment facilitates the rearrangement of trace elements within the gem, ultimately leading to a more visually appealing stone.

The other options do not accurately reflect what happens during heat treatment. The cooling process is secondary to temperature exposure and occurs after heating. Chemical alterations refer to changes that affect the chemical composition, which heat treatment typically does not do directly. Physical shaping involves the cutting and polishing of stones and is a separate process from heat treatment.