Essentially, the "sparkle" quality of a diamond depends upon the way the gem has been cut. Diamonds are cut for maximumx sparkle with consideration being given to flaws within the diamond. These flaws in the diamond could affect its sparkle in the following way. The reflection and refraction of the light and the way it plays inside the gem is responsible for the sparkle, so the sparkle of a diamond depends entirely upon the light and the path it takes in to and out of the diamond.
So when diamonds are being cut, the things given consideration to increase sparkle are the surface that should give good reflection of light and the angles within the diamond that "bounce" the absorbed light around. Since diamonds are not living specimens they cannot have their own light so the work of the diamond cutter--the lapidary is essential in maximizing these qualities to create a diamond with lots of "sparkle".
The critical angle is the largest angle at which light rays inside a diamond can escape.
The more a material bends light, the higher the RI Refractive index, the smaller its critical angle. Because diamond has a high RI, it has only a small critical angle, 24.5 degrees. This is the reaon why well-cut diamonds can be so brilliant : with smaller critical angles, there is less opportunity for light to exit, so there is more chance that the light reflects off the diamond's inner surfaces several times before finally exiting through its crown (where you will see it). Most, but not all of the light leaves it like that, it depends on the angle of incidenece and degree of polarization. Light rays that reflect back into the diamond are called secondary rays, and they contribute to a diamond's overall brilliance, fire and scintillation. Some don't leave the diamond but lose energy or dissipate into the crystal's structure.
On top, diamonds are harder, so take a better polish on their facets, so they will reflect more light also than glass (glass windows will not reflect as much as diamonds) : luster is also better, images including light will be reflected better.