Heating either lightens or darkens the basic tonality of the stone, depending on its primary condition. It can also improve clarity a great deal, though this is not always the outcome. Some inclusions can so slight they can be leached out of the gemstone by heating it, sometimes impurities within the gemstone can be dealt with in the same manner. This is a risky endeavor since different gems react differently to heat, and while some withstand very high temperatures, others may crack or explode. As long as the treatment is successful, and as long as it's permanent, this will not affect price adversely.