Sparking Brilliance of Diamond:

The total internal reflection of the light is used in polishing diamonds to create a sparking brilliance. By polishing the diamond with specific cuts, it is adjusted the most of the light rays approaching the surface are incident with an angle of incidence more than critical angle. Hence, they suffer multiple reflections and ultimately come out of diamond from the top. This gives the diamond a sparking brilliance.
Light cannot easily escape a diamond without multiple internal reflections. This is because:

Explanation

Its critical angle with reference to air is too small