Why does a diamond shine ?

Explanation

The brilliance of diamond is due to total internal reflection of light. `mu` for diamond is `2.42`, so that critical angle for diamond air interface is `C = 24.4^@ (from sin c = 1//mu)`. The diamond is cut suitably so that light entering the diamond from any face suffers multiple total internal reflections at the various faces, and remains within the diamond. Hence the diamond sparkles.