It is rammed earth.

I'm not certain what you mean -- what is rammed earth? where can I get it?

Its not a decorative finish. Its part of the building structure, like a concrete wall, except it is Rammed Earth. That entire fireplace was cast, 8" at a time, rammed by hand down to 4", then repeated until the full height was achieved. That is what gives it that stratified geologic look. Its an ancient building technology that has been around for a few thousand years and has made a resurgence in modern structures in the last few decades for its beauty and its thermal properties.