The process of applying an alkaline liquid, that is fired to become permanent, is not just a superficial process. The liquid percolates into the stone (sometime unpredictably). It also changes the crystallography of the stone, and can result in some weakness. So, when areas chip away, the design is still there. And, in some instances, the entire lines fall way, leaving a recessed pattern. (This is why the process was mistaken for "etching").

Jamey