In gemology, refractive index is one of the chief means of identifying a gemstone. It is measured using an instrument known as a refractometer. The refractive index (often abbreviated RI) is an optical property that refers to the ratio of the speed of light in a vacuum to the speed of light through a transparent medium. Light is bent as it moves from air to another material, an effect familiar from observing how a stick partially immersed in water appears to be bent. Gems with a higher refractive index tend to show more brilliance, since light is bent at a higher angle when it passes into the material. More light is then reflected back out the crown to the eye, rather than passing straight through the gem. Most gemstones have 2 refractive indices, since light is split into 2 beams travelling at different speeds when it enters the gem. This phenomenon is known as birefringence or double refraction, and it is expressed as a value that indicates the difference between the two RI’s for the gem material. Blue sapphire are classified as a double refraction. Some gemstones are singly refractive and have only 1 RI. Singly refractive gems include diamond, spinel and all the garnets, as well as other cubic crystals like cuprite, fluorite and sphalerite. Amorphous gems — those with no crystal structure — are also singly refractive.