The basic reason for the extraordinary sparkle of a suitably cut diamond is that

The distinctive sparkle (brilliance and fire) of a well‑cut diamond arises primarily from its very high refractive index. Diamond’s refractive index is about 2.42, so light entering a facet is strongly slowed and bent; this produces large-angle refraction and a high fraction of internal reflection back out through the crown when cut correctly, maximizing white light return (brilliance) and dispersion into spectral colors (fire) [2]. Optical performance of gemstones is directly linked to refractive index: higher RI yields greater light return and perceived sparkle, which is why RI is the key factor rather than hardness, mere transparency, or cleavage planes.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > QUESTIONS > p. 150
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.2 The Refractive Index > p. 149