OPTICAL EFFECTS
The earth consists of elements made up of countless atoms. Most of these atoms are in orderly, solid arrangements.
Optical Effects in Gems: Gemstone Refraction Index
Table of Opticks c1728, Cyclopaedia of Arts and Sciences
Color, Dispersion & Reflectiveness ...
Optical Effects
Some gemstones exhibit extraordinary optical effects known as phenomena. These rare and beautiful effects very often add value to gemstones.
The occurrence or non-occurrence of certain optical effects, which in the eyes of the professionals lessen the beauty and value of the stone because they have a negative influence on the "life", is considered to be decisive for the proportions grade.
Gems that show special optical effects are known as phenomenal stones. Chrysoberyl dominates this category, because not only is alexandrite the most spectacular color changing gem, cat's-eye chrysoberyl has the most dramatic eye.
Some gemstones are cut in the traditional dome shape ‘cabochon’ that helps in bringing out the special optical effects such as star and cat’s eye seen in gems like jade, star sapphire, star ruby, opal etc.
Some gems display rare special optical effects called 'phenomena', which enhance the attractiveness and value of the stone. These gems are studied for chatoyancy, asterism, iridescence and aventurescence.
Gemstones other than diamonds are often cut as Cabochons, a smooth, facetless cut which usually takes the form of a polished dome or "sugarloaf" shape. Gemstones which possess unusual internal optical effects, ...
The massive fifth edition of this classic work begins with a description of diamond and moves onto other gems and ways to identify them, including measuring their hardness, optical effects, and specific gravity, among other methods.
 See also: Stone, Gemstone, Color, Crystal, Jewel
  
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