Explanation:

Diamonds sparkle when light enters them. Depending on the type of cut and the workmanship of the cutter a diamond can give off more or less sparkle. The diamond cutter cuts exact angles into the stone which makes the diamond sparkle. Diamonds have a high refractive index--which means they can really bend light rays. So light rays that shine into them will stay inside the diamond longer than they would inside glass, plastic, or any other material with a lower refractive index. And diamonds are cut to maximize this light capturing effect. All those sides and angles are carefully designed to keep incoming rays inside a long time--by making them bounce off as many walls as possible.