Speaking from experience here.
I've found hundreds of those rings.
They are what is left of "mood rings" after they get water logged and crusty.
Around here, they sell for about $0.50 new at the schools, and since my hunting grounds are mostly schools and parks... you can imagine that I've found more than my share from kiddie drops. you have to find them in the first couple days, otherwise the "mood" band gets corroded.
Just so you know it's not fluff, this is my box from my first year detecting... Those two containers (and a third beside it on the left, that has aluminum rings) have about 100 rings. All Junk. The whole top segment are mood rings. I don't bother to keep them anymore, they've gotten tossed in years 2-4.
Oh, and while I may toss them, I still love to find them. LOL Something just deeply satisfying about pulling a ring out of the ground!
Skippy