What Unheated Means, and What It Costs

Most sapphire and ruby on the market has been heated. The practice is roughly two thousand years old, and it is honest work when it is declared. Heat completes what the ground began. The stone that never needed the furnace is the exception, and markets price exceptions.

The premium for an unheated stone of equal face is real. Thirty to fifty percent for fine sapphire, more for Burmese ruby at the top of the scale. You are paying for rarity, and you are paying for proof. The proof belongs to the laboratory, never to the seller. Gübelin, SSEF, AGL and GIA will each state, in their own careful words, that a stone shows no indication of heating. Those words are the asset.

For investment, the premium is almost always worth paying. The certificate line is the part of the stone that has appreciated fastest for thirty years, and supply of certified unheated material shrinks every season. For a piece that will be worn and loved, judge with your eyes. A superb heated Ceylon sapphire beats a mediocre unheated one at half the money.

What heat does inside the stone

Corundum goes into the furnace for two reasons. The first is clarity: the fine rutile silk that clouds a stone dissolves at temperature, and a milky crystal comes out transparent. The second is colour: heat changes the state of the iron and titanium that carry blue, so a pale or greyish sapphire can come out a deeper blue, and a ruby with a blue core can lose it. Some material is heated with a flux that melts into the fractures and heals them, leaving glassy residues behind. All of it is permanent and stable. Declared, it is legitimate work with two thousand years of history. Undeclared, it is the most common fraud in the trade, and it is committed with a smile.

How the laboratory knows

Nobody calls heat by eye. A laboratory reads the inclusions: silk that has melted or snowballed, discoid tension fractures around zircon and other crystals, fissures healed glassy, and supports the reading with spectroscopy. The wording it then uses is careful and varies by house. No indications of heating is the phrase to want. Low-temperature heating leaves less behind, and that is where reports hedge and where two houses can return different opinions on the same stone. For an expensive stone, that is a reason for a second report, not a reason to trust the first one more.

Where the premium goes

The unheated premium is not spread evenly. It is small on commercial goods, because commercial goods are not scarce either way, and it is enormous at the top, where an unheated stone of fine colour and size is one of the rarest things the ground produces. It widens with weight. And it lives entirely on paper from houses the resale market accepts, which for the top of the market means Gübelin, SSEF, GIA or AGL. An unheated stone without that paper is priced as heated by everyone who might one day buy it from you.

My own rule is simpler. I buy the stone first and the word second. Then I send the word to Lucerne and make it prove itself.

Questions collectors ask

How can a laboratory tell that a sapphire or ruby has been heated?

By reading the inclusions and supporting the reading with spectroscopy. Heat dissolves rutile silk, leaves melted or snowballed needles, opens discoid tension fractures around crystal inclusions, and can heal fissures with glassy residues. Low-temperature heating leaves less behind, which is where careful reports hedge and two houses can disagree.

Is a heated sapphire a bad stone?

No. Heat is old, permanent, stable and honest when it is declared, and a superb heated Ceylon sapphire beats a mediocre unheated one at half the money for a stone that will be worn. What is bad is a heated stone sold as unheated, which is the most common fraud in the trade.

How much more does an unheated stone cost?

For fine sapphire of equal face, roughly thirty to fifty percent more; for Burmese ruby at the top of the scale, considerably more. The premium widens with size and has been the fastest-appreciating part of the price for thirty years, because the supply of certified unheated material shrinks every season.