An optical instrument consisting of a combination of lenses which produces a magnified image of an object. Some of the more important uses of magnification: inclusion study (natural vs. man-made; mode of formation/crystal growth); surface examination(quality of cut(symmetry), polish(conditions of facets, firemarks); detection of composite stones; identification of artificial treatments; detect vulnerability to damage (e.g., presence of fracture and/or cleavage); detect doubling (SR vs DR, also may indicate amount of birefringence).

Reference: Gemwow Gemstone Glossary