applying marble or granite over brick fireplace

i have a brick fireplace and will be attaching a wood mantle.
we want to apply marble or granite sheets to the brick face of the fireplace (the exposed parts to the inside of the mantle).
looking inside the fireplace, you'd see the white firebricks; the ends of the outside layer of the brick; and then the finished side of the vertical piece of marble or granite.
those end pieces of brick - leaving them as is would reveal that this is basically a remodel of marble over brick.
it was suggested that we leave a 3/8" reveal when we attach the marble/granite (meaning the marble would stick out beyond the brick by 3/8") and then fill in that space (from the reveal angled back to the white firebrick) with a layer of heat resistant furnace cement (we'd add some texture by adding sand to it). we were told the color would basically match the firebrick and would be a cleaner look than the old red brick.
is this something that's often done? does it make sense? thanks in advance.

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