Not all rubies and sapphires respond favorably to heat treatment. Heating ruby-sapphire using any high-temperature furnace under specific conditions certainly will produce some alterations in the stone. Some of these alterations may enhance the overall appearance of the stone, while in other cases the desirable results will not be achieved. In heat treating ruby-sapphire, two important factors should be firmly established:
a) Development of proper heat treatment technology and
b) Development of furnace technology.
Both technologies are taught in our training course in gem treating rubies-sapphires. More information in my books The Heat-Treatment of Ruby & Sapphire, Beryllium-Treated Rubies & Sapphires and Flux-Enhanced Rubies & Sapphires.