Obsidian observed • Earth.com Obsidian observed

In the course of his research, MacDonald has made some intriguing discoveries in Yellowstone. For instance, in 2013 he came across a piece of obsidian, a type of glass formed from volcanic activity. Obsidian can be fashioned into extremely sharp knives and arrowheads, which was hugely useful for the Native Americans who came across it. Photo credit: Andrew Geiger/Getty Images

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