Most of them are glass, and break accordingly. A few are plexiglass over a reflective surface, and those also break accordingly, just under plastic rules rather than glass rules.

Generally speaking, any mirror can be broken with mundane force except mirrors that have turned black because a shallow reflection came out of them. Those are impervious to damage until the entity that climbed out of them is destroyed (at which point they can be broken normally though they remain flat black even as shards). Mirrors that are black because a more real double came out of them can be broken but remain black (as though their escaped "occupant" was already destroyed).