I was camping last night at a farm in Brown County, using a pit that we have built fires in for years. I placed one of the rocks that rings the pit near some coals so I could rest a grill on it for cooking. About 2 hours later...POW!!! There were pieces of hot rock that scattered about 6 feet from the fire, fortunately no one got hit. The way the rock split, it looked like it was some type of shale. I guess there was some trapped moisture and the added heat led to the explosion. My question is, are certain types of rock safer for using near a fire? Has anyone else had an experience like this?