Theo gave Senku a puzzled look. "Material? They'd be made of rock-material," he said, then decided that showing him would be better than trying to explain it. Easier too, since he didn't have the technical terms that Senku would understand from a scientific perspective. "And yes, it would be. I'll make sure to have my wand at hand." A simple augamenti would take care of any spreading fire.
"All right, if you're sure it won't interfere with the experiment, I'll see to the rocks." He looked around and spotted a medium-sized one just a bit off the path. He summoned it, then enlarged it a little with another quick twist of his wrist, so that it would be a decent barrier to any spreading fire. "I'll duplicate this one. Twelve right? A few doubling charms should do the trick."
He set the rock down, and then with a practiced swish of his wand there were two, then four, then eight, then sixteen rocks on the ground. "A few more than twelve, but the forest won't mind a few extra rocks when we're done." At least, he didn't see why it would.