Question

Why does a diamond shines?

Answer 1

Diamond has many faces. The critical angle for diamond is very less. The refractive index is high.  

The light rays get totally internally reflected from many faces and come out. Thus light energy is not lost during the refraction, reflection and total internal reflection. Thus it appears very bright. Light from multiple directions get reflected , so it appears dazzling bright.

Answer 2

it is because of the high refractive index.actually diamond does'nt shines.it reflects the light which falls on it.diamonds are cut like that they can reflect the light millions of times which falls on it.and the refractive index is also very high approximately 2.417.so,the diamond shines brightly