Explanation:

Refractive index of diamond is very much large and structure of the diamond is such that when light enters inside the diamond, there is a 'total internal reflection' which happens in the diamond, i.e., light gets reflected from one internal surface of the diamond and further reflect from other parts of the surface. This phenomenon gets repeated many times. It is because of this phenomenon the diamond shines. Diamonds are recognized for their spectacular brilliance, mostly due to the complete internal light reflection. The critical angle for the diamond and air interface is very small, so whenever the light enters a diamond, it will likely be completely reflected internally. In nature, diamonds rarely get display the brilliance they are known for. It is the technical ability of the diamond cutter that brilliantly sparks out diamonds. By suitably cutting of the diamond, many internal reflections can be made to a cutter that made the diamonds so brilliant. By suitably cutting the diamond, multiple internal reflections may happen.