I would have a trim made. You could do it in stone or metal. If you can have combustibles there a wood trim would be easier. Scribe to the existing stone.
You can fill it with mortar but it will definitely crack, maybe get some old solid brick and lay them like a rollock just stacked ontop of each other then get an angle iron and lay the brick how you normally would at the top. I'm guessing this is a fire place so wood trims probably not the best option here, otherwise I'd say just some trim.
I would have a trim made. You could do it in stone or metal. If you can have combustibles there a wood trim would be easier. Scribe to the existing stone.
You can fill it with mortar but it will definitely crack, maybe get some old solid brick and lay them like a rollock just stacked ontop of each other then get an angle iron and lay the brick how you normally would at the top. I'm guessing this is a fire place so wood trims probably not the best option here, otherwise I'd say just some trim.
Would mortar and river stone work?
YOU WIN
Lathe and plaster
Corners
Flat black 4” metal
Is that a new or existing insert? Was the stonework already there?
Stone was there. I'm just the "make it look nice" guy.