Effects of mixing lava rock with coal

I've searched all over the internet, and haven't been able to find an
answer to this question, so I'm hoping that someone here may be able
to. My question is this:

How would mixing lava rocks with coal affect such things as the
combustion rate, efficiency, etc. (as compared to just coal)?

I can make a few guesses, but I'm hoping someone here will be able to
answer with more confidence than my guesses.

Thanks!
Mark
.

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