Stones?

Recently I purchased some wood charcoal for cooking in our Hibachi pot. I was reaching in for a handful of wood and found this rather large, heavy piece.
Funny thing is it won't burn! It is a stone, a rock. It must have been in the mixture of charcoal wood for a long time as it is blackened nicely just like the wood pieces... it weighs 1.2 Lbs.
I believe the bag is sold by weight, so that means I am missing at least a tenth of what I paid for... stoners probably filled this bag!