Heat treatment is accepted as a common practice and has little effect on value, where other treatments have considerable impact. Some rubies are worth as little as 5% of similar appearing, untreated gems.

Some of the treatments are very subtle, invisible even with a loupe or microscope. To make matters worse, many treated rubies and sapphires will have natural inclusions. You have to examine them very carefully to tell if they are treated or not. Taking the value of these gems into consideration, finding evidence of treatment can be critical.

Heat Treatment

Most corundums have rutile inclusions that are in the form of long thin crystals. These are so fine they are referred to silk.

Rutile melts at a temperature slightly below that of the surrounding corundum. By heating it to the proper temperature and with a controlled cooling period, the rutile becomes absorbed into the crystal lattice. This improves the clarity and often the color of a gem. The process can also be used to enhance the silk, which improves asterism.

Heat treating is done to almost all rubies and sapphires. It sometimes leaves stress fractures, or halos around included crystals. You may also find partially dissolved silk,…