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hahahaha putting rocks on coffee? that would be quite an experimentation. If you wanna try that charcoal coffee, you can use sour wood as it is one of the most common wood used for that charcoal coffee.

I have pine and fir.

Using hot rocks from near a campfire to boil water is a technique used occasionally. A small rock from the fireplace heating a cup of coffee isn't completely outrageous. Only mostly.