Obsidian to the Greeks was no more than a semi-precious stone, black and shiny, suitable for mirrors or exotic ornaments. But to their predecessors in the Aegean through five millenia it was an important raw material for the manufacture of tools and weapons. In this article, the Aegean sources are first considered together with those in neighboring lands which may be of relevance. The characterization of the material from these sources follows on the basis of trace-element analysis, with reference also to the appearance of the obsidians and the extent to which it may be used as a criterion. The last section is an archaeological interpretation of the results.