Strange ring found in Central America

I currently live in Costa Rica, an old man recently gave me this ring. He said he found it 50 years ago in the dirt near to where there used to be a small lake which is now a football field here in Monteverde Costa Rica and that it was completely black before he polished it. The feel of the gold is of a different quality than I have felt before. If any one has any information about this ring it would be appreciated.