I put some hematite sand on the top of a glass plate that was suspended about 5 inches from the bottom of the microwave oven. I wanted to see what kind of pattern the standing waves would heat the hematite to (this microwave didn't have an RF stirring fan).
Instead of some standing wave pattern, I ended up with a fractal like antenna that grew from a molten blob in the middle. The initial molten blob extended into arms of molten material, with each arm necessarily extending in the direction that maximized RF absorption, causing additional material to melt. It grew brighter and brighter as it extended, reached some peak, then some of the arms shorted and it dimmed. Then the plate shattered from the heat.
I wrote some software that converted the picture into antenna elements for some free antenna simulation package I found at the time, but it couldn't support enough elements to get close to the shape of the antenna and would just crash beyond 10 or so.
I'm not sure how useful of an antenna it would be, since it was maximized to generate heat, but it was a neat kitchen experiment.