It's worth looking up Count Rumford on Google if you've not heard of him. It was over 200 years ago that he experimented with designs for heating with wood in an open fireplace and figured out the best proportions for creating efficient radiant heat. His design has never been significantly improved upon.
In this stone retrofit, the more efficient proportions of a Rumford fireplace will require laying firebrick to create a shallow firebox with canted sides and a fireback that slopes forward.
My firebricks are inter-fingered into the dovetailed cornerstones which are all shaped at about 105 degrees instead of 90. Stones form the outer corners where there is very little heat buildup. This creates a much better look than firebrick quoining.
We are using black 2 inch firebrick and black tinted mortar (which makes for a less 'new looking' firebox). The black mortar joints are recessed in the stonework to look like they're dry laid while the brickwork in the firebox is pointed flush.