Jackie recently commented that she had noticed that some diamonds sparkled really well even in dim lighting. She asked why some do and some do not.
Sparkle in a diamond is a function of its cut, which is a matter of proportions and not shape. A well cut diamond will return light to the viewers eyes much like mirror would.
An ideal cut diamond will have the most sparkle in any lighting condition. It will also have the most sparkle in a low lighting situation. That is provided that the diamond is clean.
The following diagram shows how a well cut diamond returns light and how a deep, or shallow poorly cut diamond leaks light and does not return it to the viewers eyes.
