A refractometer measures the angle at which light rays refract (or bend) as they move through a gemstone. The measurement (R.I. or Refractive Index) is read from a scale seen though the eyepiece of the refractometer. The R.I. is used to help identify gemstones. However, it cannot distinguish between natural and synthetic stones. It is used most easily with stones that have at least one flat, polished surface. Most refractometers can read to 1.82. They are limited by the R.I. of the hemicylinder and contact liquid. Stones with a higher R.I. than the hemicylinder or liquid are called OTL (over the limits.) Diamond, some diamond imitations, and certain varieties of garnet are example of OTL stones. Whole books are devoted to the use of refractomers and refractive index in gemstones. The instructions below will get you started with the basics. To use a refractometer, you need both the instrument itself and a bottle of contact liquid (also call R.I. liquid.)