Diffusion treatment is heating with a difference. Ordinary heating rearranges what is already in the crystal. Diffusion adds something from outside — titanium, chromium, beryllium — by packing the stone in a powder containing the element and heating it for long enough that atoms migrate into the corundum. The result is colour that was not there before, concentrated near the surface.
Surface (titanium) diffusion
The older form, used since the 1970s. Pale or colourless sapphire is heated in titanium- and iron-bearing powder. The elements penetrate a thin layer — a fraction of a millimetre — and turn it blue. The interior stays pale.
Because the colour is a skin, it follows the shape of the cut stone. On facet junctions and the girdle, where cutting after treatment or polishing has removed some of the layer, the colour is thinner or absent. Immersed in a liquid of similar RI, the stone shows its colour as an outline of the facets rather than as a body colour — the definitive sign.
Repolishing a surface-diffused sapphire removes the colour.
Beryllium (lattice) diffusion
The more recent and more troubling form, identified in the early 2000s. Beryllium is a small atom and penetrates much deeper — often throughout a small stone — at high temperatures. It produces vivid orange, yellow, orange-pink ("padparadscha-like") and enhanced blue in corundum that started pale or unattractive. Because the colour can go all the way through, immersion may show nothing.
Detection of beryllium diffusion requires chemical analysis (LA-ICP-MS or similar) for stones where the colour is uniform. Bench laboratories can sometimes detect it by the characteristic colour concentrations at the rim on partially penetrated stones, by unusual colour combinations, and by inclusion features typical of the very high temperatures involved — but a definitive negative requires instruments that few routine laboratories possess. Honest reports say when a beryllium determination was not possible.
Why it matters
Diffusion is treatment of the most consequential kind: the colour that gives the stone its value was manufactured, and in the surface form, it is fragile. A surface-diffused blue sapphire sold as heated (a permanent, body-colour treatment) is a serious mis-sale; sold as unheated, worse. Beryllium-diffused yellow and orange sapphire sold as simply "heated" is common in the market and represents a large gap in value.
What the laboratory does
Immersion. The stone is placed in a cell of methylene iodide or a similar high-RI liquid and viewed against a white background. Surface-diffused colour shows as concentration along facet edges and girdle, with a pale interior. Natural or heated body colour shows as zoning that follows the crystal's growth, not the cut.
Microscopy. Signs of very high-temperature heating — melted and altered inclusions, "chalky" fluorescence, surface residues — support a suspicion of diffusion.
Spectroscopy and chemistry where available, for beryllium.
Statement of limits. Where beryllium diffusion cannot be excluded, the report should say so rather than imply the stone is merely heated.
What a report should say
For surface diffusion: natural corundum, colour induced by surface diffusion — the colour is a treatment and is not permanent through repolishing. For beryllium: indications of beryllium diffusion or beryllium diffusion not excluded (advanced testing required). A report that identifies a vivid orange sapphire as "natural, heated" without addressing diffusion may be right, but the buyer should ask how the question was examined.
Frequently asked questions
Is a diffusion-treated sapphire natural?
The corundum is natural. The colour is not — it was added by treatment. Both facts belong on the report.
Can diffusion be seen with a loupe?
Surface diffusion sometimes shows as colour concentrated at facet junctions, but immersion makes it obvious in a way the loupe does not. Beryllium diffusion is not visible.
Does repolishing remove the colour?
For surface (titanium) diffusion, yes — the layer is thin. For beryllium diffusion, usually not, because the penetration is deep.
Which colours are most often diffused?
Blue (titanium), and orange, yellow, orange-pink and padparadscha-like colours (beryllium). Vivid yellow sapphire at a modest price is a category where beryllium diffusion is common.
Can a routine lab certify "not diffused"?
For surface diffusion, yes, by immersion. For beryllium, a routine laboratory can note indications or their absence but cannot exclude it without chemical analysis. A report should be clear about which.