[nabilhat]

McMaster also sells discs of tool steel and glass bearing balls... and carbide, and ceramic, etc. What if the bearing ball was carbide, even harder? What would happen if you reduced the hardness of a steel ball, which usually only requires normal kitchen oven temperatures? What happens if the materials switch places, say a hardened tool steel disc and glass ball?

Anyway, lots of options to safely experiment with material science concepts in very accessible and tweakable ways! No beryllium or foundry equipment required. I do think the massive, well fitted, rigid surface backing up the disc is important to constrain what this is demonstrating - plastic vs elastic deformation.