TIR ( Total internal reflection) is the main reason for brilliance of diamonds. The critical angle of a diamond is very low (24.4 degree). So if a light ray enter a diamond it is very likely to undergo TIR. which makes diamond shine.

Diamonds are known for their brilliant shine and sparkle. This is due to several factors, including their unique physical properties and the way they interact with light.

Physical Properties of Diamonds:
- Diamonds are made up of carbon atoms arranged in a crystal lattice structure. This structure is extremely rigid and allows light to pass through without being absorbed.
- The carbon atoms in diamonds are tightly bonded together, resulting in a high refractive index. This means that when light enters a diamond, it slows down and bends, causing the light to change direction.
- Diamonds are also known for their high dispersion, which refers to the ability of a material to separate white light into its spectral colors. This dispersion is responsible for the rainbow-like flashes of color seen in diamonds.

Reflection and Refraction:
- When light enters a diamond, it undergoes both reflection and refraction. Reflection occurs when light bounces off the surface of the diamond, while refraction occurs when light passes through the diamond and changes direction.
- The high refractive index of diamonds causes light to bend significantly as it enters and exits the stone. This bending of light is what creates the brilliance and sparkle of diamonds.
- The angles at which light enters and exits the diamond are crucial in determining how much light is reflected and refracted. The cut of a diamond, specifically its facets and proportions, plays a vital role in maximizing the diamond's brilliance and fire.

Internal Reflection:
- Another important factor in the shine of diamonds is internal reflection. This occurs when light enters a diamond and reflects off its internal surfaces before exiting the stone.
- The facets of a diamond act as mirrors, reflecting the light back and forth within the stone. This internal reflection enhances the diamond's brilliance and creates a captivating sparkle.

Polishing and Faceting:
- The final factor contributing to the shine of diamonds is the quality of their polish and faceting. Diamonds are meticulously polished to achieve a smooth and reflective surface.
- Facets are carefully cut and shaped to optimize the diamond's interaction with light. The angles and proportions of the facets are calculated to maximize the diamond's brilliance and scintillation.

In conclusion, the shine of diamonds is the result of their unique physical properties, such as high refractive index and dispersion, as well as the way they interact with light through reflection and refraction. The internal reflection within the diamond and the quality of its polish and faceting further enhance its brilliance and sparkle.