[Illustration: Rays of light passing into a gemstone are refracted (bent) in varying amounts depending on the gem species and also on the angle at which the light strikes the stone. The light rays are reflected back toward the top of the stone by internal faces (facets), and they are refracted again as they leave.] [Illustration: How a gem refractometer, a simple device to operate, is used to measure quickly the refractive index of a cut gemstone. A light beam passing through the opening (A) is reflected from the table of a gemstone (G) through a lens system (L) and, by prism (P), into the eye of the observer (E). The maximum angle of reflection (N), which depends on the refractive index of the gemstone, controls the angle at which the beam comes through the eyepiece (EP). The refractive index is read directly from a scale in the eyepiece.] The action of a gemstone upon the light which strikes its surface and is either reflected or passed through it sometimes results in highly desirable effects that enhance its beauty and aid in its identification. Light passing into a stone is bent from its path, and the amount of bending (_refraction_) depends upon the species of the gemstone. When the degree of bending can be measured, the gem species can be identified, since very few species of gemstones bend light to exactly the same degree. An instrument called a gem refractometer is used to determine the degree to which cut stones refract, or bend, light. The measurement obtained is the _refractive index_ of the gemstone. Many gemstones can split a beam of light and bend one part more than the other, thus producing _double refraction_, or two different measurements of refractive index. [Illustration: When a ray of ordinary white light enters some gemstones it is dispersed (split up) into rays of the separate colors of which it is composed. These rays are reflected inside the gem and are further separated by additional refraction as they leave the gemstone. This dispersion accounts for the colored flashes of light, or fire, for which diamond is highly prized.]