I stopped at the Dollar Store, and they had bags of black stones near the crafts/fake flowers. Naturally, I had to pick some up to play with.
First discovery: They are NOT all naturally black stones. They do get tumbled with a polish and coating that makes them black.
Second discovery: engraving works better than scoring.
Third discovery: You throw full power into some of these rocks, and they get warm.
Still, some of the results are quite encouraging…especially for such a cheap material.