Or they could be pockets of silicon that washed away in the rain. Of course, if they are "articicial" and were "pecked" or "drilled" into the rock surface, that would lead us to the question of who and why?

Cerrtainly, these are questions to ponder over and muse about. Indeed, why would any ancient spend time and effort at such an endeavour? To what purpose? And, to find that these are scattered over the vast geologic expanse as is the earth would be truly puzzling.

Or, they could be eroded sand as postulated above, which would lead to another question... josepp... what the fuck kinda drugs are you on?