Opus Sectile was a technique used in Rome where materials such as marble and glass were cut into definite shapes and then put in place to form a specific pattern. The individual pieces in an opus sectile work are larger than those of a mosaic and also not always uniform as in the case of mosaic.
During my last visit to Rome, I remember peeking through the open doors of the Curia (Senate House). We were not allowed inside and there was a crowd milling around the doors to capture a shot. I managed just this with my prehistoric equipment in the hustle and bustle.
Yet despite the lack of clarity, one can see clearly the way in which marble of different sizes and shapes have been put together to form this pattern.
“…But the marble floor is one of the finest surviving examples of C4 (fourth century AD) opus sectile: stylised rosettes in squares alternate with opposed pairs of entwined cornucopias in rectangles, all worked in green and red porphyry on backgrounds of Numidian yellow and Phrygian purple.”- Rome, An Oxford Archeological Guide- Amanda Claridge.