The following excerpt from the International Gem Society provides basic information on why some gemstones are heat treated. For the full article, see see https://www.gemsociety.org/article/gemstone-heat-treatments.
A Beginner’s Guide To Common Gemstone Heat Treatments
Step 2: Advanced Gemology
Gemstone heat treatments can turn mediocre pieces into items of great beauty. Heating a gem to a specific temperature and then cooling it is a common procedure in the gem world. This can improve a gem’s color and sometimes even its clarity. Such treatments do a number of good things for our budgets as well as the environment, since more rough material coming out of the ground becomes useful for its beauty. Here you’ll find a summary of the most commonly used gemstone heat treatments and tools as well as some safety tips for novice gemologists and gem cutters.