Total Internal Reflection
A diamond is designed to be a light trap. Because of its incredible density, light that enters a diamond slows down and bends sharply. If the diamond is cut correctly, the light hits the inside walls at an angle that prevents it from escaping out the bottom. Instead, it bounces off the inside walls over and over again.
The Rainbow Exit
As the light bounces around inside, it splits into its rainbow colors, much like it does in a prism. This is called 'dispersion.' When the light finally finds the right angle to exit through the top, it comes out as a concentrated 'flash' of colorful fire that we see as sparkle.
The Importance of the Cut
If a diamond is cut too shallow or too deep, the light will 'leak' out of the sides or bottom, making the stone look dull and dark. Gem cutters use complex math to figure out exactly where to place each flat surface, or 'facet,' to make sure every ray of light that goes in comes back out to the viewer's eye. [Image comparing ideal cut vs shallow cut vs deep cut in gemstones]