Every month I give sapphires to my lab to certify that a sapphire is unheated, which is essential for my business, and important to my customers especially when they buy more expensive gems. But determining that a sapphire is heated is not as easy as one might think.
First of all, what is the purpose of heating a sapphire? Heating can improve the clarity of a gem, remove zoning, and intensify color. With high enough temperatures (about 1700 degrees Celsius) you can also melt the silky inclusions in a sapphire. When sapphires are cut, the friction created by the cutting wheel subjects the sapphire to some heat already (though in most cases, at that point the sapphire has already been heated) but this is rarely if ever detectable in a lab.
How do you determine heat treatment? The first test to apply is to look at the inclusions of the sapphire under 10x magnification. In the simplest case you see silky inclusions and fine dusty inclusions. If those look undisturbed – read: not melted – then you can assume it is not heated. This takes some practice of course, but in principle anyone can learn to do this (having a darkfield loupe helps because it provides nice lighting from the back). Feathers and jellyfish-like inclusions are also good indicators, it is only a bit more difficult to judge whether those are undisturbed. For a neat search engine of typical sapphire inclusions, go here: http://www.ruby-sapphire.com/ftir-intrigue.htm
According to Lotus, "in this sapphire from Sri Lanka, evidence of high temperature heat treatment can be found in this moiré-patterned fingerprint. The once-lovely lacy pattern of liquid droplets is now besmirched by circular “explosions,” where the pressure from heating caused ruptures in the icroscopic negative crystals,..." (Photographed by Richard Hughes)Hughes, R.W., Manorotkul, W. et al. (2017) Ruby & Sapphire: A Gemologist's Guide. Bangkok, Lotus Publishing, 816 pp.; RWHL*.
However, this method is insufficient if the original gem is too clean to have enough inclusions in the first place: not every sapphire has enough inclusions and you need those to make your determination.
A loupe clean gem therefore requires further testing. A common second tool is a spectral analysis. Heating a sapphire at high temperatures removes water from it. A spectral graph will show the water content as a peak, and if this peak is lower, this is interpreted as the water having been eliminated by heat. (https://assets.thermofisher.com/TFS-Assets/CAD/Application-Notes/D10280~.pdf)
There is a human factor here, however, as the strictness of interpretation here may vary from lab to lab. In most if not all reputable labs, several people will look at the same gem and only if all conclude that they do not think they see evidence of heat will the gem pass as unheated. GIA and Gubelin are examples of labs that always have 2 gemologists assess a gemstone 100% independently from each other, before examination results are compared and discussed, before potentially seeking advice from additional colleagues; at those two laboratories, the examining gemologists also do not know the client's identity.
Now, let me turn back to the original question, does it matter if a sapphire is heated or not? Generally, 99% of sapphires on the market are heated, and heat treatment is standard. According to AGTA standards, however, this must be disclosed on the invoice – in part because it can and usually does affect prices. Heated sapphires can be up to 30% cheaper than unheated ones. Sapphires that do not need heat treatment are much rarer than sapphires that do – or rather, sapphires that can be improved with heat treatment are the most common.
In terms of integrity or safety for setting, however, it doesn’t matter that a gem is heated. The treatment is permanent, durable, and does not otherwise affect the gem. (Inken and I actually differ a little on this assessment but I think this is the most common view).
As to our own shop, we try to carry mostly unheated gems, to which we have very good access. Cecile Raley Designs specializes in the unusual gem – not the run of the mill stuff – so it makes sense for us to seek out unheated sapphires whenever possible.
Before closing let me add a quick note here about beryllium heated gems (industry calls this “Be-heat” and according to newer standards, this must be disclosed during a sale): this type of treatment requires heat treatment with beryllium, which reduces the blue tones in a sapphire, therefore it is used on yellow, orange or padparadscha like colors, but not on blues or pinks. To detect beryllium, a 10x loupe is usually also sufficient since Be-heat leaves characteristic inclusions that are not like the natural inclusions of a sapphire – they are described as “little blue halos” in industry.
Here’s a quick link to a very interesting article on a synthetic Padparadscha sapphire: http://www.lotusgemology.com/index.php/library/articles/327-padparadscha-or-pretender-an-unusual-pink-orange-sapphire
(Inken Krause sells antique jewelry at https://www.einhoerning-jewelry.com. She specializes in unheated ruby and sapphire.)