Try to see if its magnetic. It looks like theres a shade of green on there. Could it be silver? Either Chinese or Japanese. I couldn't find anything under Korea.
I have seen similar coins called Chinese silver taels. Many are fakes not made of silver and need to be examined closely.
It's Chinese. The second picture needs to be rotated 180 degrees. The characters: Wu San Guo Huang Di Translated: Five 3 Dynasties Emperor. Not sure what the "5" character means. I could be reading it wrong since its in formal script.
