The geochemistry of obsidian provides a unique tool for both archaeologists and geologists. Geochemical data from archaeological artifacts can be matched with those from known obsidian sources to indicate the parent source of origin for each artifact, and they also may indicate the presence of unknown geological sources for obsidian. Archaeologists are interested in locating all sources of artifact-quality obsidian and in identifying geochemical types on a scale that separates geographic obsidian sources and matches them with artifacts. Archaeologists then use geochemical data to reconstruct prehistoric trade/exchange relationships and group mobility patterns through time. Geologists are interested in obsidian geochemistry (1) to understand intraflow and interflow compositional variations in order to gain insight into magmatic and flow process and (2) to compare obsidians on a regional, or global, scale to provide insight into major problems of tectonics and crustal evolution of the earth. Geochemical data are thus crucial to both archaeology and geology, but archaeological provenance studies may be compromised by a lack of knowledge about the genesis of obsidians. Obsidian sources are of several geologic types; we draw examples from Obsidian Cliffs, Oregon, and Casa Diablo, California, Borax Lake, California, Brown’s Bench, Nevada/Idaho, to illustrate, respectively, geochemical homogeneity or heterogeneity in obsidian flows, the geologic processes of magma mixing, and chemical zonation in ash-flow tuffs and to discuss the ways each geologic situation may affect provenance studies. We emphasize that appreciation of these geologic processes is requisite to sound execution of provenance analysis in geoarchaeology.