> "so there exists a peculiar shiny field of obsidian rocks on the surface in that particular area, so what?" It's not sufficient for the surface to be shiny.

> "so there exists a peculiar shiny field of obsidian rocks on the surface in that particular area, so what?" It's not sufficient for the surface to be shiny. The retroreflector array needs a really specific configuration of mirrors to reflect incoming light from any direction back to it's source. That is rather unlikely to form naturally. Without that, it would only reflect the laser back to earth for a very small portion of moon's orbit, as it doesn't always point exactly to the same direction.