Opus incertum was an ancient Roman

Ancient Rome

Ancient Rome was a thriving civilization that grew on the Italian Peninsula as early as the 8th century BC. Located along the Mediterranean Sea and centered on the city of Rome, it expanded to one of the largest empires in the ancient world....

 construction technique, using irregular shaped and random placed uncut stones or fist-sized tuff

Tuff

Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...

 blocks inserted in a core of Opus caementicium.

Initially it consisted of more careful placement of the coementa (rock fragments and small stones mixed with concrete), making the external surface as plain as possible. Later the external surface became further plain by reducing usage of concrete and choosing more regular small stones. When the use of concrete between stones is particularly reduced, it is defined opus (quasi) reticulatum.

Used from the beginning of the 2nd century BC until the mid-1st century BC, it was later largely superseded by Opus reticulatum

Opus reticulatum

Opus reticulatum is a form of brickwork used in ancient Roman architecture. It consists of diamond-shaped bricks of tuff placed around a core of opus caementicium...

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