Gemstone Refractive Index

Absolute Refractive Index

The absolute refractive index of a medium is the ratio of the speed of electromagnetic radiation in free space to the speed of that radiation in that medium. As the refractive index varies with wavelength, the wavelength should be specified. It is usually given for yellow light (sodium D-lines; wavelength 589.3 nm). The relative refractive index is the ratio of the speed of light in one medium to that in an adjacent medium. (physics.about.com)

The ratio between the sines of the angles of incidence and refraction of a ray of light passing from some given medium usually air into the given medium. (O.E.D.)

Refraction
All materials refract light (alter its angle). The amount by which light is refracted is an important feature of every gemstone. The measure of a material's refractivity is known as its refractive index or R.I. Without going too deeply into physics, light travels at different speeds through different media. When light passes from one medium to another, its path is altered (bent) and it is said to be diffracted.