This is a "board formed" concrete fireplace. Here's an article on one of our favorite blogs that does a good job of describing the process: Board Formed Concrete - Life of An Architect

This is stunning. Thanks for that article link too. We’re currently designing a freestanding fireplace wall for the house we’re building and I love this look. Any idea the approximate cost to do precast board form concrete panels? Our wall would be about this size.