Oh, look what Curiosity found now!

Oh, look what Curiosity found now!  If you look closely at that photo, you'll see two little “craters” in the sand, each with what looks like a BB nearby.  I found this in today's MAHLI raw photo dump, so there's no explanation attached.

I'll speculate a bit.  My guess is that those are solidified drops of molten rock, splashed out from some (relatively nearby) impact crater where a meteor struck the Martian surface.  The fact that the little sand “craters” haven't filled in suggests that this happened fairly recently...