Colour zoning

Colour zoning is uneven distribution of colour within a stone: straight bands, patches or sectors following the crystal's growth. Sapphire is the classic case. Blue can sit in three or four sharp bands through a stone that is otherwise nearly colourless, and ruby, amethyst, tourmaline and citrine do the same in their own colours. What matters is the face-up view. A skilled cutter positions the zones so the stone reads evenly from above, most often by putting the colour at the culet, where the pavilion facets pick it up and throw it across the whole face. There are Sri Lankan sapphires whose entire face-up blue comes from a single zone at the point. Zoning visible face-up lowers value. Zoning hidden in the pavilion is the cutter's craft doing its work.

On a lab report

A report will not mark a stone down for zoning, since it is not a treatment. If the laboratory mentions it at all, the words sit in the comments, "colour zoning" or "growth zoning", with no grade attached. The laboratory's interest is evidential. Strong angular zoning is a natural growth feature that synthetics do not copy well, and the pattern helps with origin work. Colour concentrated at the surface, following the outline of the facets rather than the crystal, is a different thing entirely: one of the signs of diffusion, and that goes on the treatment line.

Tilt the stone before you buy. Zones that vanish in one position flash in another, and black velvet hides pale patches that white paper shows at once. If the stone is going into a mount, look at it in the mount, since a claw covers a zone from one side and exposes it from the other. Zoning you cannot find from above is not a defect. Zoning you can find at arm's length is a discount, and one to take.