To put it ever so fancifully, it’s because they’re alive. When a jeweler, dealer, and, eventually, you, look at a polished diamond the first time, what you’re looking for is life. Not a conceit, by the way, but an actual trade term. Life in a diamond is measured by the way it handles light, and that is strictly a function of the way it’s been cut. Formerly, that was an almost entirely subjective measurement. A number of machines and software programs now calibrate this life with extraordinary accuracy and repeatability.

What’s so endearing about a diamond, and what offers you the greatest flexibility for determining value as you choose your diamond, is that diamond life—more properly known as performance—is still highly subjective. As you might value a person for any number of virtues, diamonds perform and handle light, differently, giving them personalities.

You won’t see it right away, but performance occurs (or fails to) in three ways. Brilliance is the intensity of the light you see coming out of the diamond. Simply ask yourself this question: How bright is that light? Ask it of yourself when you first see a diamond, again as you get to know it, then again after you have seen other similar diamonds.

As you ask it, bear in mind that you’re currently holding a diamond. Probably rigidly, for fear of dropping it. Worn diamonds are almost never stationary. Breathing will cause sufficient movement to make a diamond in a necklace, or stud, perform. As you hold the diamond, move it ever so slightly. Because what you’re looking for is a brilliance that continues to change, the result of what’s now understood as contrast between darker and lighter flashes of light.

Is that really important? It’s crucial as a measure both of whether this diamond was well cut and whether it will continue to have value for you. Some diamonds return a great deal of light—they’ll pop right out the moment you see them—but will lose interest quickly. They have less life, but it may take a while for you to know that of them.

Fire, or dispersion, occurs when white light is “refracted,” in essence “deconstructed” into its component colors, because a skilled cutter arranged some facets he cut into the diamond. Facets act either as portals or mirrors for light (good) or holes for light leakage (bad).

The contrast you looked for to delineate a diamond’s brilliance has two analogues with dispersion. They’re more difficult to see, and require more time with the diamond to appreciate. The first is vividness of the color flash. The second is its sharpness/duration. Both, optically speaking, are a function of the width of that flash. The vividness of a flash can change dramatically with light conditions. Face the diamond to north sunlight for optimal viewing, though to catch full vividness, try to find light that is a tad obscured—say through the leaves of a tree. If the flashes continue to be small (pin-like), it’s because the diamond’s white light return will, in essence, have blinded or dulled you to it.