Science postulates the speed of light in a vacuum to be about 186,000 miles per second. But the velocity of light is slowed whenever it is forced to interact with the electrons of a substance, whether it's a liquid, gas or solid. Generally speaking, higher density materials have greater concentrations of electrons and therefore greater capabilities to refract light. Reflectance , or the amount of light reflected from a transparent substance, can also be inferred from a material's refractive index. The refractive index compares the velocity of light in a substance to that in a vacuum.
Diamond slows light to a remarkable degree, and ranks high in refractive index.