Zoning means colour is not even throughout the crystal. Sapphire can show bands, patches, or a shift from centre to rim. In Sri Lankan material, subtle zoning is common and often visible only when the stone moves in light.
Bi-colour and parti stones carry two distinct hues in one crystal. We describe the boundary and show both zones in video. They suit designs that celebrate the split rather than hiding it under a solid mount.
Strong zoning in a single-colour stone can make the face look uneven. Some buyers like the variation; others want a flat field of colour. Our descriptions note visible zoning when it affects face-up appearance.
Heat treatment can reduce the visibility of zoning but rarely removes it entirely. Compare heated and unheated examples in the collection to see how treatment and zoning interact.