what causes the exquisite sparkle of a diamond?

Answer 1

Its because of an optical effect called total internal reflection. Basically light comes into the diamond, and the sharp angles of the diamonds cut cause the light to reflect off the inside surface, instead of escaping out the other side. Its basically like having a little mirror on each of the inside surfaces of the diamond. below is a link to more detailed info if you want it.

Answer 2

It's the way in which the stone has been cut that makes it sparkle when light travels through it.