I did an experiment to see if I could boil water in a standard Nalgene bottle with rocks heated in coals. I posted about it here several years ago. Short answer is that it works fine, but the water will get ash in it if you put the rocks in the coals. So while having a metal container is nice it isn't essential for boiling water.

I suppose you could put the rocks above the fire to heat them up (like on a grill) and that would keep them much cleaner. I'll have to give that a whirl.

Of course, ingesting ash is not harmful, and possibly even good for you if you need some carbon to absorb other bad things you might ingest. You could always filter through a clean hanky if you were worried about it.