ultimate dump score - milled sandstone

I couldn't believe my eyes when I saw these two foot square tile looking things leaning against the shed at the dump. As I got closer the surprise got better.
I've never seen anything like this before. They look like sandstone that was milled in some way. I can't even imagine how they were milled to get this surface. If this was wood I'd say they were hand cut with a curved edge chisel.
One side is fluted, I'll call that the back side. This side is insanely beautiful. I've got them store inside so the weather doesn't adversely effect them, until I figure out something to do with them. There are eight of them.