What is the cause of sparkling of diamond?

Answer 1

Diamonds have an extremely high index of refraction, the property that makes a spoon in a cup of water look bent at the point where the water meets the air. It causes light that enters the diamond come back out at different angles, and some of the light reflects within the diamond for a while before coming back out (it's the same phenomenon as when you look at the surface of the water in a fish tank from a very shallow angle below the surface and it looks like a mirror, except to a greater angle).

Light traveling from a less dense medium, like air, into a more dense medium, like a diamond, is bent toward the normal, the imaginary line perpendicular to the surface where the two media meet at the point where the light enters. Light going from a more dense medium to a less dense medium is bent away from the normal.

Also, different colors (wavelengths) are bent different amounts, which is how a prism works and why sunlight reflecting off the insides of raindrops creates a rainbow.

Answer 2

The sparkling of a diamond is caused by the diamond's ability to refract and reflect light due to its high refractive index and dispersion. When light enters the diamond, it bounces around within the facets and is reflected back to the viewer, creating the sparkling effect known as "fire."