James wrote:
What would be a good product to use to clean natural river rock that
has been blackened from a fireplace? The smut seems to only be on
the surface, but it has been there a long time.

What would you recommend, and would you use a wire brush or something
else ?

I don't know. I can tell you this, according to the CRC Handbook of
Chemistry and Physics, the only substance in which carbon is soluable is
molten iron (that's how they make steel), so searching for a chemical
solution is probably futile.