Putting real stone facade on fireplace

Home built in 1920's, plaster walls throughout. We have a fireplace in the middle of one wall of the living room, and the chimney area is a big narrowing pillar around the fireplace all the way up to the ceiling (of the first floor). See pic here:
http://www.grilloff.com/_images/ChimneyHard.jpg
That picture was taken from the top of a ladder on the other side of the room. I zoomed in some to get a clearer pic. The different colors you see are just us painting some samples to see what color we liked in the room. The SIDES of the protuberance tilt inward on their way up to the ceiling, but the FRONT does not; the front is straight up.
The black line I painted across the image is right where it stops being solid under the plaster ON THE FRONT. By this, I mean I can hit the plaster with my hand right above the stone around the fireplace, and it seems very very solid underneath (as if there are bricks, or some kind of heavy solid surface under the plaster) up until that line; when I hit the wall above that line, it sounds hollow underneath, like there's nothing there but the plaster wall.
On the SIDES, however, it seems very solid all the way up to the ceiling. I can hit the SIDES of that big chimney protuberance all the way up to the ceiling, and it feels/sounds like it's brick or something very solid. However, on the FRONT (right above the fireplace), it starts sounding hollow upward right after that black line. It's as if the chimney, made of brick, follows the curves of the sides, but tilts back away from the front starting at the black line upwards (the wall is straight all the way up on the fornt, but only sounds hollow after the line upwards. After the line, hitting the FRONT sounds hollow underneath).
We want to apply a flagstone facade all the way up to the ceiling (like how they do with those fireplaces in ski lodges, etc.), but I'm wondering if there's anything behind the hollow part of the front to anchor things into; I don't want to apply mortar and stone on the plaster wall where's there's no support underneath (this will be a facade, but we DO intend to use real stone, so even though it's a "fake" front, it'll still be actual stone, and therefore heavy enough, I assume, to not want to just build it up on nothing but a plaster wall with no real beef behind it).
So what do I do? Smash in a hole in the plaster up above the fireplace to see what's behind there? Has anyone seen a setup like the one in the picture? And why do the sides seem solid all the way up while the front does not? Is the chimney at this point really wide but not so deep? Why does it seem solid on the sides all the way up while the front does not?
Any thoughts, advice, etc. appreciated. And if any of this seems like a really dumb question, forgive me. I've only been the muscle/gopher for home projects up until now, and even though I try to pay attention and get glimpses of the expertise, I've not come across this particular situation before. In fact, I've never helped on stuff with plaster walls before, it's always been about sheetrock on new construction. And, of course, since it's my own home, I'm extremely nervous about doing ANYTHING.
Thanks again, and I hope to hear some good sound advice.
The Complete Newb
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On Mon, 23 Jul 2007 23:09:56 -0400, CompleteNewb wrote:

It's called a chimney breast. Unfortunately, other than that, your description of it has me baffled. Unless the whole thing is brick-built with a drywall lining fixed to the front for reasons of insulation (ie to protect occupants from burning a hand on the front of the chimney breast when there's a blazing fire going).
I'd punch a hole in it and shine a light in to be sure. It's no big deal to patch a hole of any size.
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That sounds like a good starting point: see what you're dealing with. A project like this needs a good plan based on sound information. You don't want 500 lbs. of stone deciding to let go at some point. If you are new at this, definitely do some background reading. I suspect the best and safest thing is to attach the stone directly to the brick of the fireplace and chimney (with appropriate embedded straps, etc.) so you will need to get the plaster wall out of the way first. -- H
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