Greenstone -- (Michigan's state gem) was formed in gas bubbles within basalt. Basalt is a volcanic rock, formed in Michigan in part during the Midcontinent Rift approximately one billion years ago. It is found more in the western Upper Peninsula, MI, USA
A Petoskey stone is a rock and a fossil, often pebble-shaped, that is composed of a fossilized coral, Hexagonaria percarinata. They are fragments of a coral reef that was originally deposited during the Devonian period, about 350 million years ago. (Wikipedia) Photo by Mark Willocks.