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I've been thinking about sharp rocks. In the Bay Area, a really good sharp rock is obsidian. It's gorgeous material, translucent and jewel-like, and it shatters like glass, which, in fact, it is. It's also critical for our understanding of the human past, which I'll explain in a minute.

There are four great obsidians in the North Coast Ranges. Some time before 10,000 years ago, Native Americans found one of them near Clear Lake. It must have completely changed their lives. Obsidian is razor sharp and easy to break, perfect for cutting tools. You can use it for nearly everything: spears, arrowheads, knives, even a small flake is often sharp enough to split reeds or slice meat. And if your blade gets dull, it's easy to re-sharpen.

Archaeologists like obsidian too, for different reasons. Volcanoes make obsidian, and we've figured out where most of the sources are and what their chemical signatures look like. If you give us a piece of obsidian, we can usually tell you where it came from. We can then look at how the stuff was traded -- sometimes hundreds of miles across many cultures.

We also like it because we can sometimes tell you when an obsidian artifact was made. When you break obsidian, that fresh surface sucks water from the atmosphere, forming a thin rind. The longer it's exposed, the thicker the rind. Obsidian from different places does this at different rates. If you know the source, and how thick the rind is, you can figure out how long ago that fresh surface -- the edge of an arrowhead or the blade of a knife -- was made. Combine that with trade, and you can discover where different people were and who they were trading with, spanning thousands of years.

This is why archaeologists ask that you leave obsidian, and all other artifacts, where you find them. In some places, obsidian is rare. If you take even one or two pieces, there isn't any way to get that information back. Join us as we explore California's amazingly rich and complex past -- leave artifacts where you find them.


With a Perspective, this is Mike Newland.