Video narration
Why do diamonds sparkle more than ordinary glass? At a glass window, light usually bends when it leaves the denser material and enters air. But change the angle, and there's a limit: the critical angle. At that angle, the ray skims the boundary. Beyond it, total internal reflection sends the light entirely back inside. Diamond's critical angle is only about 24 degrees, so more light stays trapped, bouncing through its facets and returning as sparkle. Cut and dispersion matter too.