1923 Poland 50 Croszy toned or corrosion?

coinsarefun

I this corrosion or toning?
I looked under a loupe and do not see evidence of corrosion
but I leave it to the experts.

theboz11

Looks like it sat in rusty water and got stained.

olmanjon

I agree--corrosion would stick out from the coin, while staining and toning have no size definition.

coinsarefun

When I purchased it about 3 years ago it was slightly toned light brown mostly nickel color. Now its dark red with browns in it.
Maybe it was in water before I got and sitting in the album got darker?