Just a little additional information:

My dad said it was handed down to him and he's not really sure where it came from. He seemed to barely remember how he got it. Yet, he grew up here in Southern Illinois, near Du Bois, Illinois, and this area has a long history of coal mining. The Illinois State Geological Survey maps show the whole area is pretty well covered in underground coal mines, including Du Bois specifically. I wanted to keep some objective skepticism though, as nearby is also Giant City State Park, which is known for some pretty amazing concretion rock formations.