What Roman concrete facing technique uses diamond-shaped bricks in a net-like pattern?

The Roman concrete facing technique that uses diamond-shaped bricks in a net-like pattern is opus reticulatum.

The name means “reticulate work,” from the Latin word for a net. Builders placed small, pyramid-shaped pieces of volcanic tuff into the surface of an opus caementicium concrete core. Their visible square faces formed a regular diagonal grid of diamond shapes.

Opus reticulatum became especially common in central and southern Italy from about the 1st century BCE into the early 1st century CE. The technique was often valued for its neat appearance as well as for protecting and finishing a concrete wall. It could also be combined with other masonry styles to create decorative color and texture.

A common mix-up is opus incertum, which also used small pieces of stone but arranged them irregularly. Opus latericium used fired bricks, while opus testaceum is the related brick-faced concrete tradition. Reticulate work can be seen at sites including Hadrian’s Villa at Tivoli.