What are recessed ethanol fireplaces built into?

When installing a recessed ethanol fireplace, the surround in which it is installed must be composed of a non-combustible material. Non-combustible means that the material is incapable of igniting and burning when subjected to fire. This includes concrete backer board, such as HardieBacker®, DuRock®, & Wonderboard®, brick masonry, concrete blocks, cement, metal, and sheet glass.