What process involves exposing a gem to rising temperature to change its appearance?

Heat treatment is a well-known technique in the gem industry, specifically used to enhance or change the appearance of colored stones. This process involves exposing a gemstone to controlled high temperatures over a specified duration, allowing for alterations in color and clarity without altering the stone's inherent structure.

The heat can cause changes in the distribution of color-causing elements, such as transitioning yellow to a more desired blue in sapphires or enhancing the clarity by removing inclusions in certain types of gemstones. Importantly, this method brings out the stone's best qualities, making it more visually appealing to consumers while maintaining its natural characteristics.

Other techniques such as surface treatment and thermal alteration may involve different methods or improvements that do not specifically focus on high-temperature exposure as the primary means of inducing change, hence distinguishing heat treatment as the correct answer in this context.