The volcanic glass called obsidian was highly prized in prehistory where ever it was found. The glassy material comes in a range of colors from black to green to bright orange, and it is found everywhere rhyolite-rich volcanic deposits are found. Most obsidian is a deep rich black, but, for example, pachuca obsidian, from a source in Hidalgo and distributed throughout Mesoamerica during the Aztec period, is a translucent green color with a golden yellow sheen to it. Pico de Orizaba, from a source in southeastern Puebla is almost completely colorless.
The qualities that made obsidian a favorite trade item are its shiny beauty, its easily worked fine texture, and the sharpness of its flaked edges. Archaeologists are fond of it because of obsidian hydration---a relatively secure (and relatively low cost) way to date the period an obsidian tool was last flaked.
Sourcing obsidian--that is to say, discovering where the raw stone from a particular obsidian artifact came from--is typically conducted through trace element analysis. Although obsidian is always made up of volcanic rhyolite, each deposit has slight different amounts of trace elements in it. Scholars identify the chemical fingerprint of each deposit through such methods as X-ray fluorescence or neutron activation analysis, and then compare that to what is found in an obsidian artifact.
For information on dating obsidian see the article on obsidian hydration. See the History of Glass Making, if that's what interests you. For more rock science on the substance, see About.com's Geology entry for obsidian.
For the heck of it, try the Obsidian Trivia Quiz.
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