By Marc Nanasi, NEI Diamonds
We know that a standard round Brilliant Cut has 57 (sometimes 58) facets, so how come when you look into a diamond, it appears that there are hundreds of flashing geometric shapes?
Those flashes are known as virtual facets, and collectively they dictate a diamond’s sparkle. When light moves through a stone, it travels a longer distance than you would expect: A ray of light enters the diamond through one facet, reflects off two different facets, and exits the stone through a fourth. Because these facets work together to direct light, slightly tilting any facet will cause a ray of light to interact with a different set of facets all together. Anyone who has played billiards knows that when trying bank a shot off of a wall, a slight angle variation has a large affect on where the ball ends up. This is why proportion and symmetry are essential to a diamond’s appearance.
To illustrate the complexity of a diamond’s light performance, try holding a brightly colored object 6-8 inches over a diamond. Notice how many virtual facets take on the color of that bright object. You will also notice that these virtual facets are not all near each other, but rather scattered throughout the diamond, and separated by virtual facets of different colors.
The fact that this bright color originates in one area, doesn’t mean that it will exit the diamond in one area. This is because light is divided and redirected each time it encounters an additional facet, the result, many virtual facets. By slowly moving the bright object, the bright color leaves some virtual facets and enters others. Doing this will help an observer notice the seemingly sporadic nature of a diamond’s light performance. This experiment will be more evident with a diamond that has a superior cut grade.
The American Gems Society Laboratory has been at the forefront for research to understand how light performs in gem stones and the resulting virtual facets. Even the AGS cut grade system is based on light performance.
The author would like to thank Peter Yantzer and Jason Quick from AGS Laboratories, whose advice was instrumental for this blog post.