(Click on the picture to return to Rock Type Lab key)
I believe that this sample came from Arizona, where there is a surprisingly (to most Americans) large amount of recent volcanic rock. On the assumption that where there is recent volcanic rock there must also be recent--and potentially imminent--volcanoes, the USGS has identified much of the Amercan Southwest as a potential volcanic hazard area. Because of its sharp edges, obsidian was highly prized and widely traded by pre-Columbian peoples from the Southwest. This sample does not react with hydrochloric acid.
N. C. Heywood maintains this page, last updated 10NOV99.