True, natural sapphires are completely colorless. The presence of trace chemicals provides color through the natural process of heat and pressure. The easiest way to improve clarity and color of low-grade sapphires is to mimic the natural process and apply heat via an oven or furnace. Heating a sapphire at high temperatures specifically dissolves inclusions known as "silk" and other impurities, making the gemstone more valuable. Heat treatment is the most desirable gemstone enhancement because it does not use chemicals to alter the stone.
Pick your natural rough stones. Sellers heat or "cook" sapphires in their rough state before handing them to a gemcutter for faceting. There is no guaranteed way to predict color changes, but according to the Gem Mountain Sapphire Mine in Philipsburg, Montana, 15 to 20 percent of heat-treated sapphires turn yellow or orange.
Choose the temperature and exposure time carefully. Heating sapphires at too high of a temperature for too long may irreparably damage the gems. As noted on the website Gemstone Buzz, cooking a sapphire at 1600 to 1800°C for two hours intensifies blue and yellow colors while removing silk. Darkening a light blue color takes six hours, and lightening dark blue color takes 20 hours.
Pick the environment or atmosphere. In a 1993 study published in the journal "Gems & Gemology," authors John L. Emmett and Troy R. Douthit found that heat-treating sapphires in an oxidizing environment created yellow colors. Cooking sapphires in a reducing environment, such as that of a combustion furnace, created blue colors.
Choose the number of heat treatments. Sapphires often go through multiple treatments, in combinations of oxidizing or reducing atmospheres, to produce the best results. For instance, the Gem Mountain Sapphire Mine uses a two-step heat treatment process, starting with an oxidizing environment. They remove stones with high quality color and then initiate a final heat treatment, which they call the "blue burn."