It’s geopolymer, cast stones. Look into the Geopolymer Institute. Many stones are also scratched up and coated with another GOP that gets carved or stamped. The GOP is compared to Roman cement that crystallizes and get harder over time. Many so called carvings are achieved using special casts, like the method of casting silver jewelry, used by native Americans. There’s the nubs too. They are where the casts had holes for lifting off the cast. Once the material starts a flow out of those holes, the material continues to flow until the block hardens. If not scraped off soon enough, there is a nub. Mind you, the gop was made from various recipes given the region’s resources and for specific purposes in the construction. Each had varying set times. I would think weather influences the process as well.

Look in to Venetian techniques for applying a marble finish. Many building in Venice seem to be demos of the various techniques artisans would learn.