Lattice diffusion introduces trace elements (e.g., beryllium, chromium, or titanium) into the surface layer of corundum during high-temperature treatment to deepen or change color. It affects only a shallow depth, so repolishing or recutting can remove the effect.
Surface coatings or dyeing add color externally but are less stable and more easily detected by wear or solvent testing. Both require clear disclosure. Gemologists identify diffusion through concentration gradients in trace-element maps and altered inclusion rims. Diffusion-treated stones trade at lower prices than bulk-colored or untreated material.