Gemstones, minerals that are treasured for their beauty and durability. A large number of minerals have been used as gems. Their value generally depends on four elements: the beauty of the stone itself; its rarity; its hardness and toughness; and the skill with which it has been cut and polished. Stones such as diamonds, rubies, and emeralds represent one of the greatest concentrations of money value. During times of war or economic disturbance many people convert their wealth into precious stones, which are transportable and more easily sold. A gem cannot always be identified by sight alone. It is therefore necessary to rely on measurement of the optical properties that can be determined without harming the stone in any way.

The gemologist uses an instrument called a refractometer to measure the characteristic property of the stone, known as refractive index, which is its relative ability to refract light. In addition, an instrument called the polariscope is employed to determine whether a gem is doubly or singly refracting. A special dark-field illuminator with a binocular microscope is employed for examining the interior of a gemstone to determine whether it is of natural or artificial origin, and to search for inclusions characteristic of a given gemstone.