What is "opus sectile"

Opus sectile is an art technique popularized in the ancient and medieval Roman world where materials were cut and inlaid into walls and floors to make a picture or pattern. Common materials were marble, mother of pearl, and glass. The materials were cut in thin pieces, polished, then trimmed further according to a chosen pattern. Unlike tessellated mosaic techniques, where the placement of very small uniformly sized pieces forms a picture, opus sectile pieces are much larger and can be shaped to define large parts of the design.