I assume you are thinking about a sealed direct vent gas unit. This is just a guess: if your firebox is deep enough, you might think about installing the insert so it is recessed just an inch or so, so that it is not flush with the stone surround. Then you would need to apply something to "fill in" the spaces. I think if the insert is recessed just a little bit -- an inch or two -- and if the material you use to fill in is unobtrusive, like a cement material that is neutral in color or ceramic tile, it should fit in nicely.

But a lot depends on the actual shape of your firebox. If it is sort of rectangular, it should work fine. But if it is arched with a curved top, it might look unattractive.

I have another idea -- should have thought of this before. When we had our insert installed into our prefab fireplace, they had to fabricate an extra piece to fit across the bottom because the insert didn't come with enough stuff to completely cover the whole thing. In our case the surround of the insert is made of black cast iron. They fabricated some steel which they powder coated in black, and it looks like it's part of the unit.

In your case, an alternative to my previous suggestion, I think the surround of the insert can be fitted on the outside (forget my previous suggestion). The only problem will be to seal the spaces where the surround abuts the natural rock. I think any competent installer should be able to handle that challenge. I suspect the answer will be to stuff some fireproof material (not asbestos, of course, but a safe product with the same insulating properties) into the cracks to seal them.

I think Jotul (the Norwegian company) makes inserts that they illustrate on their website that will give you an idea of how that will look. Although we have a Jotel insert and are happy with it, I'm not necessarily telling you to buy that brand -- just look at their website to see how an installation might play out in your case. There are lots of good brands out there, and the bottom line is the competence of the installer. I would also do a google or yahoo search on fireplace inserts and see the websites of other companies.

One more suggestion, this one unrelated: make sure you get a remote thermostat with your insert. Fireplaces with inserts get very, very hot. The thermostat comes in handy to regulate the temperature. They charge too much for them, but they're worth having.

Good luck with your project. Let me know how it turns out.