Treatment
Understanding Heated and Unheated Sapphire
What heat treatment is, why almost all commercial sapphire has had it, and when paying for the alternative is worth it.
7 min read
Heat treatment in sapphire is one of the oldest interventions in the gem trade and one of the most widely accepted. It is also the single thing most likely to be misunderstood by a retail customer, which makes it worth understanding precisely if you are going to explain it to one.
What the treatment does
Corundum is heated to high temperatures, which changes how certain trace elements sit in the crystal structure. In practice it improves colour — deepening a pale blue, clearing a milky one — and it can dissolve fine rutile inclusions that would otherwise leave the stone hazy.
The change is permanent and structural. A heated sapphire does not fade, revert, or require different care from an unheated one. This is the key point for a nervous customer: heating is not a coating, a dye or a filler, and nothing can wear off.
It is also not new. The practice long predates the modern trade, and the overwhelming majority of sapphire that has ever been set in jewellery has been through it.
Why unheated stones cost more
Scarcity, and nothing else. An unheated stone with good colour is one that happened to come out of the ground already looking the way the market wants — and most do not. The premium is for the accident of nature, not for a difference in appearance or durability.
That premium is substantial and rises steeply with quality. Two visually similar stones, one heated and one not, can differ several-fold in price.
What an unheated report actually says
Laboratories do not certify a negative directly. A report will say something like no indications of thermal enhancement — that is, the analytical evidence of heating was looked for and not found.
That is the correct and honest formulation, and it is what you should expect to see. Treat wording that sounds more absolute than that with caution.
If you are selling a stone as unheated, buy it with the report. A supplier's word, however good the relationship, is not something you can pass to your own customer. See our note on reading laboratory reports.
The treatments that are not heat
Two others circulate in the corundum market and neither is equivalent to heating.
Lattice diffusion introduces an element such as beryllium or titanium during heating, adding colour from outside the stone rather than developing what was already there. It is a legitimate product when disclosed and is priced well below heated material.
Fracture filling — with glass or another substance — fills surface-reaching breaks to improve apparent clarity. Heavily filled corundum has real durability problems at the bench and in repair. In ruby this reaches its extreme in lead-glass composites, which are a different product entirely.
These need separate disclosure. "Treated" on its own does not distinguish between them, and a buyer should not accept it as a complete answer.
What to actually do
- For commercial and production work, buy heated. It is the market, availability is good across sizes and colours, and the stones are indistinguishable in wear.
- For a significant single stone where the story matters to the end customer, unheated with a report is worth the premium — provided the premium is what you are actually selling.
- Never let "treated" stand as the full disclosure. Ask which treatment.
- Pass the distinction down the chain. A customer told only that their sapphire is "treated" will assume the worst version of it.
We state the treatment recorded for each stone we offer, and where a stone is represented as unheated we supply the report that says so. More on the species in our sapphire guide.
Have a requirement?
Send the specification and we will come back with what matches.
