Do you suppose it is coke from a natural gas process some 80 years ago? I have seen this kind of porous black/gray rock before, and I have 8 tons of it in barrels in the shed.
Try to burn it, It will fire right up if it is coke.....and the question will be answered. The NG process from days of old used to strip or burn off the volatiles and leave this product, while creating NG. If it is light in weight, like charcoal, you may well have found out your answer.