Lattice diffusion

Lattice diffusion is the general category of heat treatments in which chemical additives (most commonly beryllium, titanium, or chromium) are introduced into the corundum crystal lattice at high temperatures and produce or modify color from the outside in. The treatment is distinct from traditional heat (which uses only high temperature without additives) and from glass-filling or fracture-filling (which adds material to surface fractures rather than penetrating the lattice).

Beryllium diffusion is the dominant lattice diffusion treatment in commercial sapphire today. The process heats sapphire to 1,600 to 1,800 degrees Celsius in the presence of beryllium-bearing material; beryllium atoms diffuse into the corundum lattice from the surface inward and produce orange, yellow, or pinkish-orange color through substitution mechanisms in the corundum structure. Beryllium-diffused stones are the largest supply source of low-cost orange and padparadscha-colored sapphire on the market. Disclosed on every reputable lab report and discussed under the dedicated beryllium-diffusion entry.

Titanium diffusion is less common but exists as a treatment for surface-blue sapphire. The process introduces titanium into the lattice from the surface inward, producing a thin blue layer on the outside of the stone over a colorless or pale-colored core. Titanium diffusion is less stable than beryllium diffusion and is detectable through immersion testing — the blue layer reveals itself as a non-uniform surface coating when the stone is viewed in a methylene iodide immersion bath. The treatment is disclosed on lab reports and trades at large discounts to natural-color and traditionally-heated equivalents.

Chromium diffusion has been used experimentally on ruby and on some pink-sapphire material but is not a major commercial treatment.

The disclosure language on SSEF, Gübelin, AGL, and GRS reports specifies the diffusion type ("lattice diffusion (beryllium)" or "lattice-diffused with titanium"). The general "lattice diffusion" call without specifying the diffusing element is less common because most labs identify the specific element via LA-ICP-MS.

The trade discounts lattice-diffused stones substantially. Beryllium-diffused orange sapphire trades at 10% to 30% of natural-color equivalent prices; titanium-diffused surface-blue sapphire trades at similar discounts. The discounts reflect both the treatment intensity and the fact that the color is not native — the underlying stone may be colorless or pale-colored material that owes its commercial value entirely to the diffusion process.

The treatment is stable under normal wear — the diffused element does not migrate out under typical conditions. Repolishing or recutting a lattice-diffused stone can affect the appearance because the colored layer is at the surface; a deep recut may reveal less-colored material underneath. Buyers should not recut lattice-diffused stones without understanding the depth profile.