Heated or unheated sapphire: what actually changes

The great majority of sapphires on the market have been heated. The practice is old, accepted by the trade and has no effect on how the stone holds up over time. It still weighs heavily on value. At comparable colour and weight, an unheated sapphire sells for markedly more, because it is far rarer.

What heating does

Corundum, the mineral sapphire and ruby are made of, often contains fine rutile needles that gemmologists call silk. In quantity, they veil the stone and dull its colour. Brought to a high temperature, the stone changes. The silk partly dissolves, the colour gains depth and evenness, and the transparency improves.

The result is permanent. A heated sapphire does not lose its colour in light, or with the years.

What is not simple heating

The word “treatment” covers very different realities. This is where vigilance matters.

Treatment	What it does	What it changes for value
Simple heating	Improves colour and transparency	Common practice, accepted if disclosed
Heating with residues	Glassy material fills small fissures	Disclosed on the report, often with the amount of residue
Diffusion (beryllium)	Foreign elements alter the colour within the crystal	Value far below that of a simply heated sapphire
Glass filling	Major fractures are filled with a lead glass	Mostly concerns ruby. The result is a composite material

Diffusion can be detected with certainty in laboratories equipped with fine chemical analysis instruments. This is one of the reasons the choice of laboratory matters as much as the report itself.

How the laboratory decides

The gemmologist first examines the stone under a microscope. Intact silk and crystal inclusions that have stayed sharp point to an unheated stone. Fragmented silk, altered inclusions or small stress discs around crystals show that heat has been applied. Spectroscopy completes the examination.

On the report, the mention is short. For an unheated stone, it most often reads “No indications of heating”. For a heated stone, “Indications of heating”, or a code specific to the laboratory. Wording varies from one laboratory to another, and it is the full text of the report that counts.

Should you prefer an unheated stone?

It all depends on the purpose.

For a collector’s piece, the unheated stone is the answer. The same goes for a stone worn according to the Vedic tradition, where the absence of treatment is generally sought.

For an engagement ring, a heated sapphire of fine colour often strikes a better balance. For the same budget, it allows a larger stone or a more intense shade.

In both cases, one requirement is not negotiable: transparency. At Maiwis, the status of each stone is stated as the laboratory establishes it, and the report goes with the stone.