Gemology
Gemstone Cut & Proportion
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.
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.
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.
It might look similar to this
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.
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