Brilliance of diamond is due to

The brilliance of a diamond is primarily due to the phenomenon of total internal reflection (TIR). Diamond has a very high refractive index of approximately 2.42 [1]. This high refractive index results in a very small critical angle, roughly 24.4 degrees. When light enters a diamond, it is likely to strike the internal facets at angles greater than this small critical angle, causing the light to be 'trapped' and undergo multiple internal reflections before emerging. Diamond cutters exploit this by shaping the stone into a 'brilliant cut,' ensuring that light entering from the top (crown) undergoes TIR at the pavilion facets and is reflected back out through the top toward the observer [1]. This concentrated return of light creates the characteristic sparkle and brilliance [1]. Other phenomena like interference or diffraction do not play a primary role in this specific optical effect.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > QUESTIONS > p. 150