Heated vs Unheated Sapphire: What the Difference Means for Price

By the Byrex Gems team, Toronto · Published 6 October 2026

What heat treatment does, how GRS, SSEF, Gübelin and GIA report it, why unheated sapphire commands a premium and how that varies by colour, size and origin, and which treatments must be disclosed.

Two blue sapphires of the same size, colour and clarity can sit side by side on a tray and still be priced far apart. The difference is often a single line on the laboratory report: one stone shows indications of heating, the other does not. For jewellers and designers buying sapphire, understanding that line, what it means, how laboratories reach it and how the market prices it, is basic to buying well. This article explains what heat treatment does, how it is reported, why unheated stones command a premium and where that premium is greatest, and which other treatments must be disclosed.

What heat treatment does

Heating is by far the most common sapphire treatment. Stones are heated in controlled furnaces to change their colour and clarity. The American Gem Trade Association describes heated sapphire as heated to produce, intensify or lighten colour, or to improve colour uniformity and appearance, and rates its stability as excellent (AGTA Gemstone Information Manual). In practice heating can deepen a pale blue, lighten an overly dark one, remove unwanted tones and dissolve fine rutile needles (silk) that make a stone look cloudy.

Heating is standard and accepted in the trade, provided it is disclosed. GIA puts it plainly: heating is an accepted treatment for sapphire, but for fine-quality stones, independent confirmation that there is no evidence of heat adds to rarity and value (GIA). Both halves of that statement matter. A heated sapphire is a natural sapphire, and a fine heated stone is a perfectly sound purchase. An unheated stone of the same appearance is simply rarer.

How laboratories identify heating

Heating leaves traces inside the stone. Gemmologists look for altered inclusions, such as silk that has partly dissolved or crystals surrounded by stress fractures, and for residues in fissures. They support what they see under the microscope with spectroscopy and, where needed, chemical analysis. When the evidence is clear, the report says the stone has been heated. When no evidence is found, the report gives an opinion that the stone shows no indications of heating.

That opinion is expressed in each laboratory's own wording:

• Harmonised wording. The Laboratory Manual Harmonisation Committee, whose members include GIA, Gübelin and SSEF, recommends "no indications of heating" for unheated corundum and "indications of heating" for heated stones (LMHC Information Sheet 7). Our guide to gemstone laboratories explains who the members are.
• Low-temperature heating. The same LMHC sheet provides for "indications of (low temperature) heating" when some inclusions show heat alteration while others appear unchanged. Gentle heating is harder to detect than high-temperature heating, which is one reason laboratories sometimes differ.
• GRS. GRS reports unheated corundum as "no indication of thermal treatment". For heated stones it uses H (heated, no residues), H(a) to H(d) for increasing amounts of residue in fissures, with H(d) describing significant lead-glass filling, H(Be) for heating with light elements such as beryllium, and PHT for heating under pressure (GRS).

Whatever the wording, a finding of no heat is an expert opinion based on the evidence available at the time of testing. It is not a guarantee. For an important stone, buyers often look for a report from a laboratory whose findings on heat the market trusts, and sometimes a second report.

Why unheated sapphire commands a premium

The premium comes from scarcity. Heating has made fine-looking sapphire far more plentiful than nature alone would allow, because large quantities of pale, cloudy or uneven rough can be turned into attractive stones. A sapphire that shows the same colour and clarity with no treatment at all belongs to a much smaller supply, and collectors, high jewellery houses and connoisseur clients are prepared to pay for that rarity and for the certainty that the stone is as it came from the ground.

The size of the premium is not fixed. It moves with the factors that drive sapphire value generally, and it is widest where untreated material of fine quality is scarcest:

Colour
How the unheated premium tends to behave: Largest for the most sought-after colours, such as rich, velvety blue and fine padparadscha, where unheated stones of top colour are rare. Smaller where colour is pale or where heating is less important to the colour.

Size
How the unheated premium tends to behave: Grows with size. Small unheated stones are found far more often than large fine ones, so the premium in calibrated sizes is usually modest compared with the premium on a fine stone of several carats.

Origin
How the unheated premium tends to behave: Strongest where the origin itself carries a premium and is supported by a report, for example Kashmir, Burma (Myanmar) and Sri Lanka. Unheated stones from these sources are among the most valued in the market.

Quality
How the unheated premium tends to behave: Widest for fine, clean, well-cut stones. A commercial-quality unheated stone with visible inclusions or dull colour may command little more than a heated stone of similar appearance.

Documentation
How the unheated premium tends to behave: Depends on the report. A no-heat finding from a laboratory the market trusts supports the premium; a stone sold as unheated without a recognised report is valued with caution.

Two practical points follow. First, compare like with like. An unheated stone should be measured against heated stones of the same colour, clarity, cut, size and origin, not against heated stones in general. Second, the premium is not a reason to prefer any unheated stone over a fine heated one. A beautiful heated sapphire will often make a better piece of jewellery than a mediocre unheated stone at the same price.

Other treatments to watch

Heating alone is accepted and stable. Some other treatments change a stone far more and sit at a much lower level of value:

• Beryllium (lattice) diffusion. The stone is heated with an added element, usually beryllium, which diffuses into the crystal and changes its colour. It is widely used to produce yellow, orange and padparadscha-like colours and can also alter blue stones. GIA notes that lattice-diffused sapphires generally cost less than stones heated without added colouring agents, or untreated stones (GIA). AGTA classes diffusion separately from heating and states that failing to disclose it in advertising, promotional literature or commercial documents violates the FTC Guides (AGTA). GRS reports it as H(Be).
• Lead-glass filling. Heavily fractured corundum is filled with a lead-rich glass that greatly improves apparent clarity. AGTA lists lead-glass-filled ruby and sapphire as composite materials that require special care, and GRS reports significant filling as H(d). Because of those care requirements, tell your setter before any bench work on a filled stone, and never price one as an ordinary heated stone.
• Heating with residues. Many heated stones show small residues in healed fissures. Minor residues are common and accepted; larger amounts are graded on the report, as in the GRS H(a) to H(c) scale, and affect value.

Always read the full treatment comment, not only the headline. "Heated" and "heated with beryllium" are very different stones at very different prices.

Disclosure duties

In the United States, the Federal Trade Commission's Jewelry Guides state that it is unfair or deceptive to fail to disclose a gemstone treatment that is not permanent, that creates special care requirements, or that has a significant effect on the stone's value. The guidance applies to sellers at every level of the trade, disclosure should be made before the sale, and for online and catalogue sales it belongs in the description of the stone (16 CFR 23.24).

In Canada, the Competition Act makes it unlawful to advertise or market a product in a way that is false or misleading, and the Competition Bureau administers it (Competition Bureau). Describing a heated stone as unheated, or leaving out a treatment that affects value, risks falling foul of that rule.

For a jeweller, the practical rule is simple: pass on exactly what the report says. Keep the report with the stone, describe the treatment on your invoice and in any online listing, and do not describe a stone as "natural" in a way that implies it is untreated when it has been heated.

A short checklist before you buy

1. Read the treatment line on the report in full, including any codes or comments on residues or diffusion.
2. Check which laboratory issued the report and verify the report number on the laboratory's own website.
3. Confirm that the weight, measurements and shape on the report match the stone.
4. Note the report date, and consider a recent report for an important unheated stone.
5. Compare the price with heated stones of the same quality, size and origin, not with sapphire in general.

Unheated sapphires from Byrex Gems

Byrex Gems has supplied fine coloured gemstones and natural fancy colour diamonds to the jewellery trade since 1988, and we are members of AGTA and CJA. Every stone we offer states its treatment, and many of our unheated sapphires come with reports from GRS, SSEF, Gübelin, GIA or other leading laboratories. To see them, use the Unheated filter in Stone Search, or browse Unheated Sapphires. Approved trade buyers see prices online; to apply, open a trade account.

To go through a report with us before you buy, email info@byrex.com or send a WhatsApp message to +1 (437) 450-3786.