I wanted to clarify a few things about treated sapphires for myself and decided to pass on what I discovered. This is a quick and very basic layout designed for anyone without formal jewelry training does not have too much scientific terminology.
A natural (untreated) and truly magnificent sapphire is rare. It is estimated that less than 1% of natural sapphires are gem at the time they are mined. The rest undergo some type of treatment to become the brilliantly colored stones you see for sale! These treatments of lower grade gemstones alter their color and clarity.
Sapphires with silk (minute, needle like inclusions) can be heated to extremely high temperatures which causes the silk to dissolve. After the stone is properly cooled what remains is an exquisite clear blue gemstone. Heat treatment is a widely accepted practice and permanently alters the sapphire.
Referred to as "surface diffusion", pale or colorless stones lacking silk are packed in beds of chemical powders. They are then exposed to very high temperatures until a thin layer of color appears on the surface. The color is permanent but only on the surface of the stone which means it can be worn away, polished away or if the stone is re-cut, the colorless interior will be exposed.
This is what it sounds like - a radioactive treatment that makes a sapphire more appealing in color and clarity. The color fades quickly a stone could be radioactive, so this treatment is rarely used.
Specimens with surface fractures can be polished with colored oil. The fractures allow the stone to absorb the oil and it effectively hides cracks and improves color. Again, the alteration to the stone is temporary so it is not a widely used method.
Is Value Affected?
Treatments generally increase the value of poor stones, but the type of treatment used and the resulting quality of stone play a role in the final value.
Whether you are certain you want a natural sapphire or you don’t mind a treated stone if the price is right, your best bet for finding quality stones of significant size is to partner with an antique dealer.