This article examines both the principles behind the functioning of the scanning electron microscope-energy dispersive spectrometer system and its application to the chemical and structural analyses of gems. The examples presented describe the distinction of natural from treated opals, the relationships observed between a diamond and its kimberlite matrix, the identification of inclusions in ruby and taaffeite, the elemental mapping and chemical analysis of maw-sit-sit, and the study of chemical interrelationships between various types of garnets. (Abstractor's note: the text indicates that maw-sit-sit is a mottled green, black, and white ornamental stone from Mogok, Burma, first described by Gübelin in 1964.) -- AATA