You’re sitting in your 5rd grade science class when a glint of sunlight makes its way from the window to a triangular paperweight on your teacher’s desk. Mysteriously, almost magically, the sunlight is cracked apart and lays its raw colors on the wall. To look at a prism, all you see is a clear pyramid. To understand why one color is splashed into a spectrum, requires some science.
This angular wizardry is going on inside of a diamond, too. An experienced craftsman takes a rather dull looking lump and cuts it just right. Cut it too shallow, and light slips through the bottom. Cut it too deep, and it takes a bounce and heads for the side door. When the light gets away, the diamond never gets a chance to throw back a jaw-dropping flash. And that’s what you want.