construction site

Day to day, I pass by a construction site with a very large and colorful crane. There is a big and deep hole like for a basement floor. I noticed of late that work on the building itself has started. The workers and the crane are building what might be a stairwell. What is new to me and of interest is that it is being assembled of very large, rectangular slabs of stone or something heavy like stone with a few notches and rectangular holes and such. Each segment is perhaps the height of the stairwell; they seem to fit together like giant pre-fab pieces for a three-dimensional jigsaw puzzle.