Colour zoning

Uneven distribution of colour within a stone: straight bands, patches or sectors following the crystal's growth, common in sapphire. What matters is the face-up view. A skilled cutter positions the zones so the stone reads evenly from above; zoning visible face-up lowers value, while zoning hidden in the pavilion is the cutter's craft doing its work. Tilt the stone before buying, since zones invisible in one position can flash in another.