The recent application by Burton and Krinsley (1987) of back‐scattered electron (BSE) petrography to obsidians from sources located in the south‐western United States established that this method can effectively resolve and characterize included micro‐crystalline phases that have proven difficult to analyse by optical thin‐section microscopy. In the first extension of their original study, we have examined, using BSE petrography, obsidians from island sources located in the Mediterranean, including sources known to have been exploited in prehistory. Because of the kinetic contrasts on their crystallization, these microcrystalline phases reflect the magmatic history of the obsidian, providing information about superheating, supercooling, sub‐solidus processes, and other phenomena. This information is of significance for the chemical analysis of Mediterranean obsidians and also as the basis for a powerful alternative to existing non‐destructive analytical methods for the sourcing of archaeological and art‐histor