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.

Try using acetone. That is what the installers used on ours after they were done. I noticed they didn't get everything, so I went out and bought some and just put it on a sponge and 'scrubbed' away. Just make sure you have plenty of ventilation, because that stuff sure does stink!

Emrit, I stole a slate tile fireplace for your room. Actually, the original photo was a bit too gray, so I warmed it up a bit. You'll have to get tile samples and bring them into your room to see how they look in your light.

I planned on SW Collonade Grey for the walls/ceiling, a “custom” blend walnut stain on my red oak floors and the same stain on the Alder beams above. Mantel will be the same walnut stain too. Sounds lime a lot of stain, but the kitchen is tile floor with white oak cabinets. What stage is your build - can finish selections be changed at this point? If so, I would recommend a consulation with Maria Killam (https://www.mariakillam.com) to ensure your finishes will be cohesive and provide the proper background for the overall look and feel you want to acheive in your new home. Your fireplace is fine, but will be a bossy element to work with (start reading Maria's blog posts and you'll see what I mean).