Not much really to say about it, I saw it from afar and for a split second I thougth it's a slice of some gemstone, like those agate geodes, mounted as a decoration.
Which would be next to impossible given its location. It's not the kind of a place to put expensive artwork/craftwork or something. It would be shattered to pieces in no time by some local chaotic neutral citizens after a few beers, and pieces would be soon looted, if not by the vandals, then by local kids. I'd give it a month at most.
Enough of a surprise to walk closer and take a look, and ... it turned out to be a rusty plate, with some scraps of a blue foil that probably not very long a go was some informational sign.
I think I can imagine how it happened - grains are finer towards the center, more coarse at the edges, so that part had more time to corrode. So probably it started with water getting in between the foil and metal at the edge, and slowly proceeded inwards, and the foil gradually broke off from outer 'rings' towards the inner part.
How cool is that? :D I just can't stop grinning looking at it, both out of the sheer silliness of thinking 'gemstone slice' and because it's just plain boring chunk of rusty metal looking awesome.
adventure time
You see a rusty plate. Will certainly bring to halt a bird dumb enough to try to fly through it, or a stone of a decent size, but not much more than that. Looks heavy for its size. Can be smelted for raw materials or sold as scrap metal. Common item, trash, craftable, not enchanted. Resists electricity. Weak to acid and water. +2 DEF; -1 AGI; -1 CHA if wielded a shield or worn as a cloak.
Obvious exists are: left, back, up and over the fence