Heat treatment is accepted as a common practice and has little effect on corundum values. In contrast, other treatments have considerable impact. Some can reduce the worth of rubies and sapphires drastically, by 50% to even 95%.
Some of the treatments are very subtle, invisible even with a loupe or microscope. To make matters worse, many treated rubies and sapphires will also have natural inclusions. You have to examine them very carefully to detect any treatment.
Due to the impact some corundum treatments have on value, finding evidence of these procedures can be critical.
Most corundums contain rutile inclusions in the form of long, thin crystals, so fine that gemologists refer to them as “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 enhance the silk, which improves asterism.
Almost all rubies and sapphires receive heat treatment. It sometimes leaves stress fractures or halos around included crystals. You may also find partially dissolved silk, distinguishable…