The first measure of a diamond's sparkle is brilliance, or how well it reflects white light. When you see a diamond sparkling in a dimly lit or candlelit room, what you're seeing is its brilliance. If a diamond isn't cut correctly, it will appear dim and dull, even in perfect lighting.
The second measure of a diamond's sparkle is fire, or how well it displays colored light. When you see a diamond with a rainbow-like appearance, it's a result of the correct balance of proportions from the top and bottom of the diamond. Diamonds with too deep or too shallow a cut won't refract colored light properly, resulting in a flat appearance.
The last measure of a diamond's sparkle is scintillation, or how well the diamond reflects light when it's moving, when the light source is moving, or when the person wearing the diamond is moving. When a diamond has high scintillation, it has very few spots where no sparkle occurs. In a well-cut diamond, most of the surface area of the diamond emits some visible sparkle.
Finally, polish affects the ability of a diamond to absorb, refract, and reflect light. The more polished and transparent the diamond, the more sparkle it will have. We polish our diamonds at 100X magnification, 10 times the industry standard, so they're as beautiful and bright as possible.