"Walter R." wrote in
:

I am putting wood planking in my living room. Some of the planks will
butt against the fireplace hearth, which consists of petrified wood
set in mortar. The area where the stones meet the concrete pad is
very irregular due to the irregular shaped stones. The floor is a
concrete pad.

For a neat installation and in order to provide for some expansion,
I will need to undercut the fireplace hearth by 1/2" so that I can
slip the ends of the planks under the stones.

http://tinypic.com/view.php?pic=48zmrsh

I have a 4" electric grinder. Will that be suitable for cutting a
1/2" undercut into the rocks? What kind of blade(s) should I use? I
bet this is going to be a very dusty affair!?

Rent a jamb saw from the local shop. Many places won't rent to you if they
know you're doing a fireplace, so keep looking. Use two shopvacs, one on
each side of the blade. This will keep the mess to a minimum.