Lattice diffusion zoning is the spatial pattern of colour in a gemstone that has been treated by diffusion of dopant atoms (typically beryllium or titanium) into the crystal from an external source. The zoning records the migration path of the dopant from the surface of the stone inward and serves as one of the most reliable optical indicators of diffusion treatment when the depth of penetration is limited.

In a faceted stone, dopant atoms diffuse inward along all surfaces simultaneously. The treated layer therefore conforms to the outer geometry of the stone after cutting, producing colour rings or zones that follow the facet pattern rather than the natural growth structure of the original crystal. Under immersion microscopy, with the stone bathed in a high-refractive-index liquid (typically methylene iodide), the conformal nature of the zoning becomes evident: colour saturation peaks at the girdle, at facet junctions, and in surface-parallel layers, while the interior remains pale or differently coloured.

This is in contrast to natural colour zoning, which follows growth-related axes (commonly aligned with the c-axis in corundum) and bears no relationship to the post-cutting facet pattern. A sapphire that shows hexagonal colour bands aligned with crystal growth is exhibiting natural zoning; a sapphire that shows a continuous coloured rim conforming to the cut shape is exhibiting diffusion zoning.

The distinction matters because diffusion treatment commands a substantial price discount in the trade. Surface-localised diffusion (titanium in sapphire, for example) is generally detectable by immersion examination alone. Through-stone diffusion (beryllium in corundum, particularly in stones treated for sufficient duration to allow full penetration) does not produce visible zoning and requires chemical analysis (LA-ICP-MS, LIBS) to confirm.

For day-to-day trade examination, lattice diffusion zoning is checked by immersing the stone in methylene iodide under a gemmological microscope, viewing through the pavilion, and looking for the characteristic colour-conformal pattern. A jeweller without laboratory access can spot many diffusion-treated stones with this single test, although stones treated to full depth must be sent to laboratories such as GIA, Gubelin, SSEF, AGL, or Lotus Gemology for chemical confirmation.