Fossil fungus (I presume you had toadstool in mind) is not likely - extremely low potential for lithification.
I note the pale thin crust (damaged) on the middle part, an the circular mark on the other side. Grasping at straws, I wonder if it is waste from a casting process involving liquid metal or a cold setting material such as plaster of Paris. I have in mind a casting mould connected to a pouring basin by a conduit, and what we have here is the residual casting material set in the basin. The circular mark would come from where the sprue (the material filling the conduit) broke off. With that in mind, can you tell us if it is metallic? (a magnet would indicate iron/nickel metal/ore). It could also be useful to know its density.
In terms of material, it looks most like slag. That could still be related to metal smelting/casting, but I can't quite see how this particular object would come to be made of slag.
Your local history museum might be able to help: they should know what industries operated in the area in years gone by and may even have seen such an object already.