Here’s a fun story about the Crater of Diamonds State Park in Arkansas:
An extinct volcano in Arkansas hosts the only public diamond mine on Earth [Ars Technica] – “The 37-acre search field near Murfreesboro sits atop an ancient volcanic pipe that erupted roughly 100 million years ago. That eruption brought diamonds that formed deep within the Earth’s mantle to the surface, where they now wait in the soil for anyone with a garden trowel and patience.”