Obsidian is very useful, because it will hold a very sharp edge, so it makes good knives and arrowheads and fishhooks and so forth. Even today some surgeons still use obsidian knives in eye surgery.
In the Stone Age a lot of people wanted obsidian tools, because they were sharper than flint. But you could not find obsidian just anywhere. So one of the first things that people traded for, when they were just beginning to trade, was obsidian. Even as much as 200,000 years ago, in the Middle Stone Age, people were already carrying obsidian as much as a hundred miles away to trade. In the Late Stone Age (the Neolithic) the reason the town of Catal Huyuk (in what is now Turkey) was so rich, about 8000 BC, was that they were also selling obsidian.
Learn by doing: check out some obsidian at a rocks and gems store
More about stone tools: flint
More about the Middle Stone Age
Archaeology : Uncovering the Mysteries of Our Past, by Richard Panchyk (2001). With twenty-five projects, like counting tree rings, and serializing cars from photographs.
Lithics (Cambridge Manuals in Archaeology), by William Andrefsky (1998). A specialist’s manual for studying all kinds of stone tools.
Production and Exchange of Stone Tools : Prehistoric Obsidian in the Aegean, by Robin Torrence (1986). Unfortunately out of print.