The stones are the debris from the gas line rupturing prior to the explosion. The line is under pressure, the pipe blew out and the pressure thru up the debris. you can see the cloud of gas following the debris, the when a spark from the debris ignites the gas, you can follow the ignition of the gas cloud. This pipe was over 50 years old and had no corrosion protection, similar to over 65% of the gas lines buried throughout the US.