Greenstone is an informal name for a form of pumpellyite, also called chlorastrolite. Pumpellyite is named for Raphael Pumpelly, who was a geologist who visited and wrote about Keweenaww rocks and minerals early in the 20th Century. In Michigan, greenstones form inside the bubble holes in lava flow tops, they coat the bubble walls and fill many of the holes. Then on the beaches of the Keweenaw or isle Royale they weather out, and the harder pumpellyite separates from the softer basalt. You can also find greenstones on mine dumps where the amygdaloid was mined for copper. Central and Delaware are two places where greenstones are on rock dumps. The south shore of Isle Royale is a good place for beach smoothed greenstones.