A surface covering comprises multiple units adapted to mate with one another and cover a surface. Each said unit comprises at least one primary rotational tessellation element that defines the sides of the unit, has an irregular configuration, includes a face having molded surface irregularities or variations therein, and has at least two pairs of irregularly shaped sides. Preferably, gaps of variable width are defined between visible edges of adjacent units in the surface covering. The surface covering has a natural appearance such that a repeating pattern is not readily apparent. A structural wall unit system is also provided.