Been using that expression for sixty years. Can’t lay claim to any special cleverness. It’s just another way of saying “ashes to ashes, dust to dust.” From clay we came and to clay we return. And, what is a rock if it isn’t some clay under pressure? One could just as easily say “stardust.” That’s true too.
Depends how much carbon is in that clay or rock....
The material our flesh is made from is irrelevant, as no flesh shall enter heaven. It’s the living spirit, the life force given to the flesh by God that is what ultimately matters.
Rocks don’t have that living spirit in them, so they are not my cousin.