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Gemstone Cut & Proportion

ideal proportions in gemstones We saw on the page titled Light in Gems that, depending on the gem's critical angle, when light strikes an external faceted gem surface one of two things will happen.

Light will either reflect off the surface or it will refract into the stone.

We also learned that light which has refracted into a gem and strikes an internal facet surface will either refract or reflect again. If light inside a gem refracts, it leaves the gem. Ideal proportions for each gem type (based on refractive index) will be slightly different

Faceting a Gemstone

Cutting a gem to proper proportions makes light behave as we wish. In a well-proportioned gem. A high percentage of the light entering this gemstone will return to the eye. A gem cut to these proportions will show maximum brilliance.

illustration of the behavior of light in a well proportioned gemstone

Gem Cut Too Shallow

But if a gem is cut too shallow, much of the light will refract out of the pavilion of the gem rather than reflecting internally. That will make the gem seem washed out and windowed.

illustration of the behavior of light in a gemstone cut too shallow

Gem Cut Too Deep

A gem cut too deep will be much too dark. Again, the light has refracted out of the pavilion.

illustration of the behavior of light in a gemstone cut too deep

Gem Cut Off Center

If the gem has been cut with the pavilion off-center the path with light returning to the eye will cause one half of the gem to show brilliance while the path with light leaving the gem will cause the other half of the gem to show extinction.

illustration of the behavior of light in a gemstone cut with the pavilion off center

It might look similar to this

illustration of the appearance of a gemstone cut with the pavilion off center

Gem With Excessive Pavilion Bulge

Colored gemstones are usually step-cut as a means of retaining carat weight. The deviation from a straight plane on the pavilion is called the bulge. An excessive bulge can ruin what may be an otherwise attractive gemstone.

illustration of excessive pavilion bulge

A gemstone cut with excessive bulge will show both windows and extinction.

illustration of the appearance of excessive pavilion bulge

Keeping in mind the concept of critical angle and its affect on how light behaves in a gem will allow you to mentally lay out the infinite number of light paths in a gemstone. And remember that in a properly proportioned gem, light entering the pavilion will also reflect up through the table or crown.

Other Characteristics of a Well-Finished Gemstone

  • Girdle with even thickness, not "wavy", not excessively thin or thick.
  • No pits, nicks or scratches on facets
  • Facets with a high degree of luster
  • Facet junctions that are crisp and well aligned
  • Table is symmetrical and perpendicular
  • Good Face-up symmetry
  • A pleasing length to width ratio

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