Heated vs Unheated Gemstones: The Complete Australian Guide

What does 'heated' mean for a gemstone?

A heated gemstone is a natural stone, mined from the ground, that has been put in a furnace after mining to improve its colour, its clarity or both. The stone is not melted or reshaped. The heat rearranges what is already inside it: trace elements that cause colour, and tiny needle-like inclusions that make it look cloudy.

Heating is the oldest and most widely accepted gem treatment. People were heating carnelian and other quartz thousands of years ago, and gem traders in Sri Lanka and Southeast Asia have heated sapphire and ruby for centuries. Today it is so common in some species that an unheated stone is the exception, not the rule. Across the coloured gemstones you will see in a jeweller's case, heating sits alongside other gemstone treatments such as oiling, irradiation and fracture filling, but it is the one buyers ask about most.

An unheated gemstone is one that has had no heat treatment at all. Its colour and clarity are exactly as they came from the ground, apart from cutting and polishing. That is all the word means: it says nothing on its own about how good the colour is.

What is the difference between heated vs unheated gemstones?

To the eye, a heated and an unheated stone of the same colour and clarity look the same. The differences are in rarity, price, paperwork and, occasionally, durability.

The most important point: heated does not mean lab-grown. A heated sapphire is still a natural sapphire. Laboratory-grown (lab-created or synthetic) sapphire is a different product altogether, grown from scratch in a factory, and it can also be heated. Our guide to lab-created and simulated stones explains how those are made and named.

Which gemstones are heat treated?

Heating only helps some stones. Where it works, it is routine; where it does not, it is rare or damaging. Typical practice looks like this:

Corundum, the mineral behind sapphire and ruby, is where heating matters most, both because it is so common and because unheated corundum carries a real premium. Emeralds are the opposite case: their clarity is improved with oil or resin, and heating would crack them. Star sapphires and rubies are a special case too, because their star depends on intact silk; heating that dissolves silk can weaken or remove the star, which you can read about in star and cat's-eye effects.

How can you tell if a sapphire is heated?

Usually you cannot, without a microscope and often a laboratory. Heating leaves clues inside the stone, and a trained gemmologist looks for them under 10× to 60× magnification.

• Silk: rutile needles (silk) in unheated corundum are long, sharp and continuous. After high heat they break into dotted lines or dissolve completely.
• Stress halos: mineral crystals inside the stone expand faster than the sapphire around them, cracking it into a flat, glassy disc. These discoid fractures are a classic sign of heating.
• Melted crystals: some included crystals melt and re-form with a rounded, frosted look.
• Residue: rubies heated with a flux (a melting agent) can have glassy material healed into fissures. Labs record this separately, often as minor or moderate residue.
• Colour zoning: heat can blur the sharp, straight colour bands typical of unheated sapphire.

When inclusions are not conclusive, labs turn to instruments. FTIR spectroscopy (infrared light that reads the chemical bonds in a stone) can reveal changes in hydrogen-related features; UV-Vis-NIR spectroscopy reads how the stone absorbs light; Raman and photoluminescence are used for spinel, where heating broadens certain peaks. Laser ablation chemical analysis picks up beryllium diffusion, a separate treatment where beryllium is driven into the stone during heating to create strong orange, yellow or pink colour. Our guide to how gemmologists identify stones covers each instrument, and origin determination uses many of the same tools.

Low-temperature heating is the hard case. It changes colour without leaving much evidence, so a lab may not be able to say either way. That is why reports for sapphire and ruby say 'no indications of heating' rather than 'unheated': it is a careful statement of what the lab could find.

Does heating affect a gemstone's durability?

Standard heating does not change hardness. A heated sapphire is still 9 on the Mohs hardness scale, and a heated tanzanite is still 6 to 6.5. The colour from standard heating is stable for life under normal wear and light.

Two qualifications matter. First, the heating process itself can crack a stone, and those cracks may be healed with glassy residue; a heavily residue-filled ruby is a little less robust than a clean one. Second, lead-glass filled ruby is not simply heated: it is low-grade ruby with fractures filled with glass, which can be etched by lemon juice or damaged by a jeweller's torch. That is a composite, priced and disclosed very differently.

For rings, what really decides wear is the stone's own hardness and toughness (resistance to chipping), not whether it was heated:

For more on choosing for constant wear, see durability for everyday rings and our list of gemstones suited to daily wear.

Are Australian sapphires heated?

Some are, many are not. Australia's sapphire comes mainly from the gemfields of central Queensland, around Rubyvale and Sapphire near Anakie, and from the New England region of New South Wales, around Inverell and Glen Innes. These basalt-related deposits produce sapphires rich in iron, which gives the classic inky, deep blue, plus greens, yellows and the bi-coloured parti sapphires Australia is known for.

Dark Australian blues have often been heated to try to lighten them, but their very high iron content responds less to heat than the lower-iron blue of Sri Lankan or Madagascan stones. As a result, many Australian sapphires, and especially parti and green sapphires, are sold unheated. A lab can confirm both treatment and, in many cases, likely origin. See our main guide to Australian and imported sapphires for colour and origin in more detail.

Is an unheated sapphire worth the extra cost?

It depends on what you value. Unheated stones with fine colour and clarity are genuinely rarer, and the market pays for that rarity. In sapphire and ruby the premium for a comparable unheated stone is commonly in the range of about a fifth to a half more, and can be much higher for top-colour rubies and large stones. Where heating is universal or cannot be detected, such as tanzanite, aquamarine and blue zircon, there is little or no price difference.

These are rough guides only; colour, origin and cutting move the price a great deal. Unheated is a reason to pay more for a stone you already love the look of, not a reason to accept weaker colour. A well-coloured heated sapphire usually looks better than a grey unheated one at the same price. Collectors who buy rare collector stones tend to insist on unheated; most people choosing coloured engagement rings pick on colour and budget first. No gemstone, heated or unheated, should be bought on the expectation that it will rise in value.

Is a heated gemstone still natural?

Yes. A heated sapphire is a natural sapphire that has been treated, and it should be described as a natural, heated sapphire. It is not lab-grown, not synthetic and not fake. The trade standard, reflected in the CIBJO guidelines, is that known treatments are disclosed, and under Australian Consumer Law a seller must not mislead you about what a stone is. If treatment is not mentioned, ask.

Disclosure should be specific. 'Heated' covers standard heating. Heating with residue, beryllium diffusion, lead-glass filling and surface coating are each different treatments and should each be named, because they affect price and care differently.

How should you set and care for a heated gemstone?

For standard heated sapphire, ruby, aquamarine and citrine, care is the same as for untreated stones of the same species. Warm water, mild detergent and a soft brush are safe for almost everything. Our stone-by-stone care guide lists the exceptions.

• Tell your jeweller before repairs. Retipping claws or resizing can involve a torch. Corundum tolerates this; tanzanite, emerald, residue-filled and glass-filled stones may not, and should be removed or protected.
• Skip the ultrasonic for tanzanite, emerald, glass-filled ruby and any stone with surface-reaching fractures.
• Choose a protective setting for softer or tougher-to-wear stones: a bezel or low claws suit tanzanite and emerald. Our guide to setting styles compares the options.

Heated vs unheated gemstones: how to choose step by step

1. Ask directly. Is the stone heated, unheated or unknown, and has it had any other treatment?
2. Ask for a lab report. For sapphire and ruby above about 1 carat, or any stone sold as unheated, look for a recognised laboratory such as GIA, AGL, SSEF or Gübelin.
3. Read the exact wording. 'No indications of heating' supports unheated; 'indications of heating' means heated. Check for residue, beryllium or glass filling.
4. Compare like with like. Price heated stones against heated, unheated against unheated, at the same size, colour and clarity.
5. Choose for wear. Match hardness and toughness to how often you will wear it, then pick a setting that protects the stone.
6. Keep the paperwork. Store the report with your valuation and show it to any jeweller before repairs. Unfamiliar terms are explained in our jewellery glossary.