Gem Identification (Part 4) Refractometry

This is called a Refractometer. It measures a ratio called the Refractive Index. The Refractive Index indicates the speed of light in the air divided by the speed of light inside the gemstone. When light enters a gemstone 2 things happen. The light is refracted or bent, and the light DECREASES in speed. The Refractometer measures the bending of light inside the gemstone and converts it to a numerical value. The range is 1.31-1.81 as viewed though the lens. The technique involves placing the gemstone upside or table down over a minute drop of contact liquid which is extracted from the brown bottle. This test is sometimes VERY CONCLUSIVE as in the case of Tanzanite which has a somewhat unique R.I. that is not seen in other stones. This instrument also indicates OPTICAL CHARACTER as the lens is rotated during examination. The unknown red cabochon shows a R.I. of 1.76-1.78