The vast majority of ruby sold at any price point has been heat treated. Industry estimates put the proportion above 95% for commercial material, and even fine Burmese and Mozambique ruby, stones that could sell for thousands per carat, is more often treated than not. The treatment is accepted by all major gemmological laboratories, disclosed in their reports, and does not diminish a stone’s intrinsic identity as a natural ruby. What it does affect is price: a fine unheated ruby of comparable quality will sell for a multiple of a heated equivalent.
What heat treatment does to ruby
Ruby is corundum coloured primarily by chromium. Heat treatment modifies this colour by altering the oxidation state of iron and titanium impurities, dissolving the fine rutile silk that causes haziness, improving clarity by healing fractures, and intensifying or improving the distribution of colour. Standard heat treatment uses temperatures between 1,200°C and 1,800°C. At the lower end, colour can be improved without completely destroying the silk. Partially dissolved silk is itself a diagnostic feature. At high temperatures, silk is completely dissolved, fractures are healed, and the stone may develop stress halos around crystals where thermal expansion caused micro-fractures.
How it is detected
The primary tools for heat treatment detection in ruby are microscopy and spectroscopy. Under the microscope with darkfield illumination, the gemmologist looks for: altered or dissolved rutile silk, ruptured or burst crystal inclusions, stress fractures radiating from crystals (heat halos), altered colour zoning, and healed fingerprints with residue. No treatment leaves no evidence. If the stone is completely clean internally, that itself requires explanation. Spectroscopy, particularly UV-Vis and photoluminescence, can detect changes in chromium spectra associated with heating. For high-value stones, laboratory analysis using EDXRF to detect flux residues (if glass filling is suspected) is also standard.
Glass filling: a different and more serious treatment
A category of ruby treatment that requires separate consideration is glass or lead glass filling. This treatment, common in commercial ruby from Mozambique and some other sources, involves impregnating surface-reaching fractures with glass, drastically improving apparent clarity. Unlike heat treatment, glass filling is not accepted as a minor enhancement. It is heavily disclosed on laboratory reports, significantly reduces the stone’s durability (glass softens with heat and acid), and represents a substantial departure from the stone’s natural state. A heavily glass-filled ruby should trade at a fraction of the price of a natural untreated or lightly heat-treated stone of comparable apparent appearance. The distinction is invisible to the naked eye and requires laboratory detection.
The price premium for untreated material
A well-documented, laboratory-certified unheated ruby of fine colour and quality commands two to five times the price of an equivalent heated stone from the same origin. The premium reflects scarcity, genuinely fine colour in unheated ruby is extremely rare, and collector preference for stones in their natural state. The documentation is essential: a claim of no heat treatment without a major laboratory certificate (GIA, Gübelin, SSEF, Lotus) carries no market weight. The certificate must explicitly state no indications of heating, not simply absence of evidence.
What this means for buyers
Anyone buying ruby above $500 per carat should require a laboratory report from a recognised laboratory. Below that threshold, assume heat treatment. For stones above $2,000 per carat, the no-heat premium becomes commercially significant and worth investigating. For stones above $5,000 per carat, the origin-and-treatment combination is the primary value driver, and the laboratory certificate is non-negotiable. The question to ask is not whether the stone is heated, assume it is, but what the report says and whether the price reflects the treatment status correctly.