Light slows down by nearly 60% when passing through a diamond, which is what creates its brilliant sparkle.

The speed of light in a vacuum is a universal constant of approximately 299,792 kilometers per second. When light enters a transparent medium like a diamond, it interacts with the dense electron clouds of the carbon atoms, which slows its effective velocity. This reduction is measured by the refractive index. Diamond has one of the highest refractive indices of any natural gemstone at 2.417.Because the refractive index is the ratio of the speed of light in a vacuum to its speed in the medium, light travels at only about 124,000 kilometers per second inside the stone. This represents a 59% reduction in speed. This dramatic transition causes refraction, where the light path bends significantly as it enters and exits the material.A key factor in a diamond's brilliance is the 'critical angle,' which for diamond is a narrow 24.4 degrees. When light hits an internal surface at an angle steeper than this, it undergoes Total Internal Reflection. This means the light stays trapped inside the stone, bouncing between facets until it is directed out through the top.The dispersion of light also plays a major role. In 1666, Isaac Newton demonstrated that white light is composed of a spectrum of colors. Diamond has a high dispersion value of 0.044, meaning it separates white light into its component colors more effectively than most other gems. This creates the 'fire' or rainbow flashes that characterize high-quality diamonds.