Diamond sparkle because of total internal reflection. Total internal reflection is a phenomenon that occurs at the boundary between two mediums, such that if the incident angle in the first medium is greater than the critical angle, then all the light is reflected into that medium.
Along with total internal reflection, the following are the three other factors that determine the sparkling of the diamond:
Reflection occurs when the light ray hits the diamond and bounces back, giving away an instantaneous shine. Reflection is the light that hits the diamond and is immediately bounced back up, giving it an instantaneous shine. While this glimmer is impressive, it’s only the very tip of the true radiance a diamond displays. Only a portion of the light hitting a diamond is reflected; the rest travels through it.
Refraction and Dispersion
As the light moves through the diamond, it is scattered and fractured, creating the sparkle that diamonds are known for. This is the refraction. Diamonds are tiny, complicated prisms; the light enters through the top and then is angled around the diamond’s inside before being aimed back towards the top and out through the surface.
Refraction and dispersion are the reasons why the light ray travels through the diamond. Diamond acts like a tiny complicated prism through which the light ray travels at different angles.