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Gemstone Cut & Proportion

Well Cut Gemstone

illustration of the behavior of light in a well proportioned gemstone

Gemstone Cut Too Shallow

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

Gemstone Cut Too Deep

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

Gemstone Cut Off-Center

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

Will Look Like

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

Gemstone - Excessive Bulge

illustration of excessive pavilion bulge

Will Look Like

illustration of the appearance of excessive pavilion bulge

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.

Critical Angle

  • When light strikes a gem surface, it will either reflect off that surface or refract into it
  • Every optical material has a unique refractive index
  • The cone you see in our illustrations represents the critical angle
  • If light strikes at an angle outside the cone / critical angle it will reflect off that surface -- whether external or internal.
  • If light strikes at an angle inside the cone / critical angle it will refract into the material
  • Light traveling inside an optical material that strikes a surface will behave in a similar manner.

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.

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.

Gem Cut Too Deep

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

A Gemstone 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.

It might look similar to this

Gemstone 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.

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

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