Obsidian
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 use obsidian knives in eye surgery.
To find out more about obsidian, check out these books from Amazon.com or from your library:
Archaeology for Kids: 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.

