Color zoning is uneven color in a gem: bands or patches of deeper and paler color.
In more detail
It is common in sapphire and amethyst. To check, place the stone table-down on a white surface; zoning shows best through the pavilion. Cutters place a dark zone low in the pavilion, so the facets spread its color across the face.
Related terms
- Sapphire: Sapphire is corundum of any color other than red. The word alone means a blue stone; other colors take a color word, such as yellow sapphire.
- Amethyst: Amethyst is the purple variety of quartz. Iron in the crystal, together with natural radiation, causes its color.
- Curved striae: Curved striae are curved growth lines or color bands in a flame-fusion synthetic. Natural corundum grows with straight, angular zoning instead.
- Hue: Hue is the basic color of a gem, such as red, blue or green. A second color can modify it, as in purplish red.
- Pleochroism: Pleochroism is the property of a colored doubly refractive stone to show different colors in different directions. It happens because the stone absorbs its rays differently.