National Geographic : 1950 Dec
© National Geographic Society 787 Ektachrome by 1. Anthony Stewart and John E. Fletcher Looking Through a Refractometer, the Mineralogist Identifies a Gem by the Light It Bends Since every known gem has its own index of refraction, Dr. G. S. Switzer, Associate Curator of Mineralogy at the Smithsonian Institution, can tell that the stone on the instrument is a kunzite. By means of reflecting mirrors, lenses, and a graduated scale, the refractometer measures the stone's index, thus identifying it accurately. Another clue is its specific gravity, or the stone's weight compared with that of an equal volume of water, determined with the balance in the background. Gems' value depends on beauty, durability, rarity. Of 1,500 minerals, 75 more or less qualify as gems.