Brilliance and Scintillation
Brilliance and scintillation are terms that are commonly used to describe a gemstone. Although they are related, they are not the same thing and there is often confusion over exactly what they mean. I hope here, to shed a little light on the issue (pun intended). They both describe how light interacts with a faceted gemstone.Brilliance is sometimes referred to as “internal luster” or “liveliness”. It is the amount of light entering a stone that is returned to the eye of the viewer. It is in that brilliance that you see the color of the gem. The amount of brilliance is dependant on the type of stone, its clarity, and how it is cut. A stone that is not well cut and proportioned will “leak” light from the back of the stone. The result is dull or dark looking areas, called “areas of extinction”, when viewing the stone face up. With many commercially faceted gemstones, if you look down through the top of the stone, you can see through the bottom. This is called a window and is not ideal. In a well cut gemstone, the light passing through the top of the stone will not pass straight through but will be reflected back and the stone will be said to have more “brilliance”. If a stone is heavily included, the inclusions can diffuse or absorb some of the light entering the stone and reduce the brilliancy, making the stone appear fuzzy. Brilliance is sometimes described as a percentage, indicating the percentage of light entering the stone that is being reflected back.
Scintillation is sparkle. It’s the flashes you see as you move a stone in the light. It is determined by the optical properties of the particular gemstone and how it is faceted. The number of facets affects the amount of scintillation a stone shows. A gem cut with a smooth cone-shaped pavilion (the bottom part of the stone), could have high brilliance, but very little scintillation.
Another related term is “dispersion” or “fire”. It refers to the light being split into the spectral colors before it is reflected back to the eye. That’s the multi-colored flickering effect that diamonds are famous for. You also see it of course, in cubic zirconia and in a few colored gems.
You will see these terms used often in describing gemstones. Sometimes they are used correctly and sometimes not. As with most things there are disagreements over the exact definitions of the words. If you would like to share your understanding of these terms or add anything that will help clarify, please do.