If you don’t have a fireproof container to boil water, you should be able to heat up rocks in a fire then add them to the water to boil it. You can even boil water in a hat with this method, as evidenced by Les Stroud. It would definitely make early game survival a bit easier.
your water would just fall off the rock since its a flat surface
you’d need to find a rock in a nice bowl shape, which is like finding a needle in a hay stack.
trying to build one out of two rocks would be tough too, and rocks require ALOT of time and heat to actually reach boiling point, not to mention extreme care to not get hot to explode in your face like frag grenades
You don’t boil the water in the rocks, you heat the rocks in a fire, and then add them to a container of water, boiling the water as the rocks cool.
Basically just think of the rock as heating kit, not a container