The question that surrounds the entire issue of heat treatment of sapphires (or other gem materials) is: "What should I buy?" Personally I believe that if you see a stone you like and that the treatment is within the norms (i.e. heating as opposed to diffusion or dyeing) you should buy it. While it is nice to be able to say that your stone is completely natural, the fact of the matter is that man plays a role in the entire process whether treated or not. The stones are mined by man and cut to achieve their maximum beauty by man. If you want something completely natural you could always get a sapphire crystal, but most of them are really pretty ugly. I have given Kathy, my wife, both natural and heated sapphires. My concern is that the stone be beautiful, well cut and pleasing to the eye. The rest of it is moot. I don't want to own an ugly stone just because it hasn't been heated.
Years ago there was very little difference in the price between natural and heated sapphires. The general public had very little knowledge of what was being done to stones before they reached a retailer. Today, thanks to organizations like the AGTA and forward thinking retailers like myself and larger firms like Tiffany's, the public has been given the right to know what is being done to their stones. This has, in fact, led to a premium (in some cases a significant one) being placed on stones that can be proven to be natural color.
Unfortunately, there are still far too many jewelers out there who either don't understand what is being done themselves, or don't care about informing their customers (in some cases illegally). The FTC states that treatment of gem materials MUST be disclosed if the lack of treatment results in a significant price difference. Regrettably they don't define "significant" but a good lawyer could argue that even a 5% difference could be construed as that. Technically what this means is that fine sapphire that has been heated will need to be disclosed but tanzanite treatment would not (because ALL tanzanite that comes on the market has been heated). Personally, however, I have always believed that ALL treatments should be disclosed as the public has the right to know exactly what they are getting and acted accordingly.
So to answer the question posed at the beginning of this post, buy what you like! Just make sure that you are informed about what it is you are actually buying.
Pictured above: Natural color sapphire.