Microwaves vibrate more than just water, it’s just that water is gonna be what’s absorbing most of it. Microwaves absolutely would heat stone, and i have direct evidence unless my microwave is just doing something entirely new to science because it frequently heats the bowls and plates to a greater degree than the food itself. it’s kinda annoying

The google lying machine suggests it heats stone through trapped moisture, also dielectric processes where semi conductive minerals absorb energy and heat up

I do think it’s an ultimately stupid idea though and the trapped moisture component IMO would make the stone very liable to just blow up if it got as hot as you’d need it to in order to take it out and sear a steak on it. Like it wouldn’t heat evenly so the thermal stress would be dangerous