I believe the rods were graphite, just like the moderators. Of course the problem with that was that by the time they were inserted the reactor was already out of control. All the graphite rods did was burn, which certainly didn’t help matters! Worse, after a while the blocks of graphite used as the moderator also burned, and was difficult to extinguish. What they needed (other than better reactor design) was a ready store of neutron poisons that would have flooded the reactor vessel before rod insertion. Unfortunately, by the time they were using neutron poisons the explosion and fire had already destroyed anything like containment.