I recently purchased a nice group of ancient coins on eBay specifically to get this one coin.
It is an AE fouree denarius of Diadumenian, the son of Emperor Macrinus (217-218 AD). All of his denarii are rare coins and I found a VCoins listing for another fouree where the seller says that fourees of Diadumenian are rare.
The style looks pretty good. The coin weighs 2.50 grams, which is about half a gram under a typical specimen, so I am going with it being a nicely toned AE fouree and not a silver denarius with a thick patina.
I thought I'd share it with the group. It is based on a PRINC IVVENTVTIS reverse with depiction of Diadumenian holding a scepter and standard, with two standards to his right (RIC, Vol. IV, Part 1, #102).
Perhaps both yours and the VCoins example are Limes denarii rather than fouree cores? I see no evidence of silver plating on either. For comparison, searching on "Diadumenian" and "limes" at ACsearch.info:
I would classify it as 'limes' especially as it has a great olive green patina without any of the corrosion or surface blemishes associated with fouree Since all the Imperial bronzes of Macrinus and Diadumenian are rare it is a good find After 45 years I am still looking for a Diadumenian I better get moving !
Debasement during the Severan period seems to have made the silver plating of counterfeits less necessary. High-tin bronze probably looked "white enough" when new and the process was simpler than that used to produce fourrées.