22k gold and cabachon emerald ring

I can see why your jeweler wasn't keen to try and date this piece. Without caratage stamps, a maker's mark, or a dinstinctive style, dating this piece is very difficult, perhaps even impossible. A jeweler knowledgeable in historical jewelry making techniques could perhaps ballpark it by examing its construction and tool marks.

I know that 22 carats is rare in jewelry, but common in 19th century coinage (such as the British Sovereign), which could indicate a 19th century jeweler melted down gold coinage to make this piece. Then again, the style is very simplistic, crude even, and given that it was found in the Gulf, near where the Spanish established their South American colonies, it could be a 17th century piece, manufactured in South American for export to Spain (or local consumption for a wealthy colonist). 

Ultimately, professional inspection by a knowledgeable jeweler or historian could mean the difference between a retail price of a few hundred (for a crude contemporary piece) and another of several thousand (for a 17th century artifact). Of course, disclosing the location of where you found it could help.