Heating corundum is older than gemmology. Sri Lankan and Thai cutters have used fire to improve stones for centuries, and today the large majority of ruby and sapphire sold worldwide has been through a furnace. Heating is a legitimate, stable, permanent treatment — but it is a treatment, it affects value substantially, and it must be disclosed. A stone sold as unheated at an unheated price, that turns out to be heated, is a mis-sale of a large sum.
What heating does
Corundum is heated in furnaces at temperatures from about 800 °C to over 1,800 °C, in controlled atmospheres, for hours or days. Depending on the starting material and conditions, heating can:
- Intensify or lighten colour by altering the oxidation state of the iron and titanium that colour sapphire, or the chromium in ruby.
- Remove blue or brown colour from ruby and pink sapphire, giving a cleaner red.
- Dissolve rutile silk into the crystal, improving transparency and sometimes deepening blue by releasing titanium into the lattice.
- Heal or partly heal fractures, occasionally with the aid of a flux, which itself becomes a filling that must be reported.
The result is a stone that is still natural corundum — the crystal grew in the earth — but whose appearance has been changed by human intervention.
Why disclosure matters
An unheated sapphire or ruby of fine colour is rare; heating makes fine colour common. The price difference between the two can be several-fold at the top of the market. "Natural" on a certificate means the crystal is not synthetic; it does not mean the stone is unheated, and buyers who conflate the two overpay.
What the laboratory looks for
Silk. Intact, sharp rutile needles in intersecting sets are strong evidence that the stone has not been heated to high temperature, because such heating dissolves them. Partly dissolved silk — needles broken into dotted lines, or with a "snowball" of altered material — indicates heating.
Tension fractures around crystals. When a stone is heated, included crystals expand at a different rate from the host and can crack the corundum around them, producing a disc-shaped fracture (a "halo") centred on the crystal. Crystals with fresh, sharp haloes are a sign of heat.
Altered inclusions. Some included minerals melt, recrystallise or change appearance at furnace temperatures. Their state is compared with the unheated reference.
Colour zoning. Heating can blur or diffuse sharp natural colour zoning.
Surface features. Very high-temperature heating can produce pitting or a glassy residue on facet surfaces that survives repolishing in recesses and around the girdle.
Fluorescence. The UV reaction of some rubies and sapphires changes with heating; it is supporting evidence rather than conclusive.
The limits of detection
Low-temperature heating — below the point where silk dissolves or haloes form — leaves little or no microscopic trace. Detecting it requires spectroscopic analysis (FTIR, UV-Vis) that most bench laboratories do not have. An honest report from a bench laboratory states its finding in terms of what was observed: no indications of heating observed under microscopic examination is defensible; guaranteed unheated from a microscope alone is not.
Some stones simply cannot be determined: a clean, silk-free sapphire with no included crystals offers nothing to examine. A report should say when a determination could not be made.
Trade terminology on reports
- No indications of heating — the examiner found evidence consistent with an unheated stone and none against.
- Indications of heating — evidence of heat treatment observed.
- Heated, with residues in fissures — heat treatment with flux-assisted healing; the residues are a filling and are stated.
- Undetermined — the stone provided no usable evidence either way.
Wording varies between laboratories; a report should define its terms or use ones that are unambiguous.
Frequently asked questions
Is a heated sapphire still natural?
Yes. Heating alters a natural crystal; it does not create one. The report should say both "natural" and "heated".
Is heating permanent?
Yes. The changes are stable under all normal conditions of wear and cleaning.
Does heating weaken the stone?
Not measurably in ordinary heating. Very high-temperature treatment can leave a stone with more internal fractures than before, which is a durability question rather than an identity one.
Can a heated stone be sold as natural?
It can be sold as natural because it is natural. It cannot honestly be sold as unheated, and the treatment must be disclosed.
Why do some reports say "undetermined"?
Because the stone had no features that reveal heating either way. That is an honest answer and better than a guess.