[scarmig]

I feel totally stupid for not knowing this. That's counteracted by what I now know is possible, though:

"Additionally, microwaves can melt certain types of rocks, producing small quantities of synthetic lava"

I know what I'm making for Thanksgiving.

[scarmig]

It also led me to find this:

http://amasci.com/weird/microwave/voltage2.html

Teaser: the title is "UNWISE MICROWAVE OVEN EXPERIMENTS"