Easy. Electroplasma is very hot and very energetic. When it ruptures out of the conduit, The hot energetic plasma not only mechanically fractures the materials around it, but the plasma itself is a form of matter that will, when it’s energy is released and it cools, return to whatever state it would normally be at room temperature.
Surface ships use deuterium and anti-deuterium as fuel, deuterium is liquid at room temperature. Assuming the combined plasma is also deuterium, that would mean it is eventually condensing to liquid. So I imagine the interaction between the plasma and some other material would turn the other material into a sort of spongy texture, which is probably dark due to being scorched. Thus, I don’t think that’s rock at all. It is scorched material from around the plasma conduit, that has been melted and integrated with the plasma which then returned to a lower energy state, namely deuterium steam or liquid.