The Role of Brilliance in Rubies
The term brilliance describes the amount of light that a gemstone reflects back to the viewer from the interior of the stone. Brilliance is a consequence of the cut of a gemstone, and it is an important characteristic, especially for rubies, as it determines the perceived liveliness and color of the gemstone.
As white light enters a ruby from above, the light will travel through the gem in a straight line. As it does, some of the colors that make up the visible spectrum are absorbed. The spectral colors that are not absorbed determine the stone’s color. So for a ruby, blue, violet, and green portions of the visible spectrum will be absorbed, leaving mostly red to be reflected back to the eye of the viewer.
When any light passes through the crown and reaches the rear or pavilion facets of a ruby, it can either travel out through the bottom of the ruby or reflect back into the stone, depending upon the angle at which it strikes the facet.
When the light passes straight through the stone, the ruby will not return any brilliance for the viewer to observe. Light reflecting off of the pavilion facets then exit the stone through another part of the pavilion. They will not return any brilliance to the viewer. Only those light rays that reflect back through the crown toward the eye of the viewer provide brightness and color. These rays account for a ruby’s brilliance .
Cutting for the Best in Rubies
A “window” in a gemstone is an area where light passes through a cut stone without being reflected back to the eye of the viewer. Even the most carefully cut ruby will show windows at certain angles, but windowing should be minimal, or completely absent when a stone is viewed from directly above. That is the sign of a quality gemstone that has been cut to highlight its best features.
An area within a gemstone that looks extremely dark is considered an area of “extinction.” These areas occur when light leaches out of the sides of the pavilion, rather than returning to the viewer. Again, the absence of an observable extinction in a gemstone is the sign of well-done cutting to the particular features of each ruby gemstone.
Many natural ruby crystals have broad, flat, tabular shapes. In order to conserve weight, these stones are often cut with very shallow pavilions and broad crowns. Light entering the crown of the stone passes directly through the flat pavilions of these shallow cut stones, creating a pale, unsaturated window. These characteristics can be observed in Thai and Cambodian rubies in particular. In the most extreme cases, cutters will fashion pavilions with steep sides and flat bottoms in their effort to conserve weight. These stones exhibit a central panel of windows and very little brilliance.
Rubies from Möng Hsu (Myanmar) are generally pyramidal or bipyramidal in shape. These rubies, if cut to conserve weight, may have deep pavilions, which can occasionally cause extinction effects. Thai stones also have dark areas, not necessarily because they are cut improperly, but because they are typically medium-dark to very dark in tone.
The common understanding is to measure a gem’s brilliance as the percentage of its face-up area that shows neither extinction nor windowing when lit and viewed from above. The accepted values for breaking down the brilliance in a ruby is based on the following level of observable windowing or extinction:
- 75 percent or more of its surface shows “excellent” brilliance
- 60-75 percent is “very good,”
- 40-60 percent is “good,”
- 25-40 percent is “fair,” and
- < 25 percent is “poor.”
Now that the understanding of the importance of brilliance in a ruby’s color and quality is clear, the next page will cover how to choose Rubies as Heirlooms.