This term is not to be confused with bi- coloured gemstones such as Ametrine or Bi-Coloured Tourmalines, which is where the Lapidarist deliberately facets the gem to best demonstrate its variation in colour. Zoning, or colour banding, which appears in gems such as Citrine, Smokey Quartz, Amethyst, Sapphire and Ruby, is more often regarded as unevenness in colour.
When zoning occurs in a gemstone, the Lapidarist will tend to orient the gemstone so that the bands run parallel with the table facet. Then, when the gemstone is viewed from the table or the facets on the crown, the colour will appear uniformed.
Normally, the darker band will be placed in the culet, as it reflects its colour throughout the gem. In fact, a technique known as foiling was once used by fraudulent jewellers to increase the apparent colour of a gemstone: a tiny piece of tinted foil was placed under the culet to exaggerate the colour throughout the whole gem.
Once a zoned gemstone has been faceted, the gem is often concealed in a bezel or channel setting so that the banding is not visible from the facets below the girdle. On the flipside, if you have a gemstone that is normally susceptible to banding and it is set in an open gallery or a Tiffany setting, and when you view it from a side profile no banding is visible, then you know you have a fine specimen of that gem variety.
