Two sapphires of the same color, size and clarity can differ in price by a factor of three or more, and the only thing separating them is a line on a laboratory report: heated, or not heated. That single determination rests on physical evidence inside the stone, read by microscope and spectrometer. This article explains what heat does to corundum at the atomic and inclusion level, and the specific techniques laboratories use to detect it.
What Heat Actually Does Inside Corundum
Heat treatment means holding a sapphire or ruby at temperatures between roughly 800°C and 1,800°C, sometimes for days. The purpose varies: dissolving the fine rutile needles that make a stone milky, redistributing iron and titanium to deepen blue, or oxidizing iron to shift an overly dark stone toward a cleaner hue. The corundum lattice itself survives — this is one of the hardest and most thermally stable minerals on earth — but everything trapped inside it does not.
That asymmetry is the whole basis of heat detection. A crystal inclusion that formed at 600°C in the ground cannot be taken to 1,600°C in a furnace without changing. Hydrogen bonded into the structure as hydroxyl groups behaves the same way. Laboratories do not look for evidence of heat in the sapphire; they look for evidence in what the sapphire is carrying.
The Microscope Comes First
Most heat determinations begin and often end with a gemological microscope under darkfield and fiber-optic illumination. The classic signatures are unambiguous once you know them. Zircon and other crystal inclusions expand faster than the surrounding corundum and blow out radial stress fractures around themselves, producing the discoid or snowball halos that trade gemologists look for first. Rutile silk that was once a crisp, oriented needle network appears shortened, beaded, or dissolved into a fuzzy cloud. Color zoning that was originally sharp and angular looks smeared or diffused at its boundaries.
Flux-assisted heating leaves its own evidence. Borax or similar flux melts into surface-reaching fissures and solidifies as glassy residue with a distinctly different lustre and trapped gas bubbles, visible in reflected light. Lower-temperature heating, however, may leave almost nothing for the microscope to find — which is where instruments take over.
FTIR Spectroscopy and the Peaks That Heat Erases
Fourier-transform infrared spectroscopy measures how a stone absorbs infrared light, and it is the workhorse instrument for heat detection. Its value lies in the hydroxyl region, between roughly 3,000 and 3,400 wavenumbers, where several diagnostic features live.
Boehmite, an aluminum hydroxide that forms along twin planes in unheated corundum, produces sharp peaks near 3,085 and 2,115 wavenumbers. Boehmite decomposes at a few hundred degrees, so its presence is strong evidence that a stone has never seen meaningful heat. Working the other direction, a series of peaks anchored near 3,309 wavenumbers is commonly associated with unheated metamorphic material, while a prominent peak around 3,161 wavenumbers is widely treated as an indicator of heating in blue sapphire. Laboratories read these as a pattern rather than a single trigger, and always in combination with microscopy.
Where Detection Gets Genuinely Difficult
Low-temperature heating is the live problem in the trade. Stones treated at 800°C to 1,000°C — enough to improve clarity slightly without destroying silk — can leave evidence so faint that two competent laboratories reach different conclusions on the same stone. This is why reports sometimes read as carefully hedged language rather than a verdict, and why a stone occasionally carries two reports that disagree.
Additional techniques narrow the gap. Raman spectroscopy on zircon inclusions measures lattice crystallinity, which heat partially restores, giving a temperature proxy. UV-Vis-NIR spectroscopy tracks iron-related absorption features near 377, 388 and 450 nanometres, which sharpen or weaken with treatment. And for beryllium lattice diffusion — a treatment that adds an element rather than just applying heat — only LA-ICP-MS can reliably detect beryllium at the parts-per-million level, because it is too light for standard X-ray fluorescence to see.
What This Means When You Read a Report
Report language is deliberate. GIA states "no indications of heating" rather than "unheated," because the claim is about detectable evidence, not metaphysical certainty. GRS uses designations such as H for heated and describes residues where present. Read those words literally — they describe what the laboratory observed with the instruments available.
For any stone where the no-heat premium is material, a report from a major laboratory is not optional documentation, it is most of what you are paying for. At Thai Gems we have worked with Thai and Sri Lankan heat treatment for decades and disclose treatment on every stone we sell. Browse our unheated sapphires, all supplied with GRS or GIA certification confirming no indications of thermal treatment, or our heated sapphires and ruby collection for stones where transparent disclosure and value matter more than the premium. Contact us for trade pricing or certification questions.