The upper peninsula is known for its lush natural resources including the state s gemstone isle royal greenstone according to the usgs the state contains 1853 mines. We often hear i thought petoskey stone was the state stone the answer is petoskey stone is the state stone and greenstone is the state gemstone geology formed after hot lava flows settled and hardened about 1 billion years ago. In 1972 the governor of michigan signed a bill designating chlorastrolite as the official state gem.