looks like a honed marble or limestone but could also be concrete. (I can't enlarge the pic to get a good look at it)

search, "stone fireplace surrounds' and you should find them.

I' sure you could have a cement artisan build one similar.

the inside of the fireplace is a specialized fire brick tile done in a herringbone. those tiles are really really expensive. I've checked!

Try Atelier Jouvence

Francois & Company makes surrounds similar to this. Their website is:

https://francoisandco.com/ Good Luck!

It's a cast material, but too fine to be concrete IMO.