A Roman construction technique involving walls of very small rough stones, not laid in courses, but held together by the mortar or attached to a concrete core. This technical term was also used by Vitruvius c 30 BC to describe the irregular patchwork stone surface that was commonly applied to Republican-period walls, as a decorative facing for the concrete inner core. Opus incertum was the most common facing for ordinary concrete walls of the 2nd and 1st centuries BC. The face of the concrete was studded with three- to four-inch irregularly shaped pieces of stone, usually tuff.