Cool!
I used to buy lots of (supposedly) unsorted, uncleaned ancient coins. Once I got really excited when upon initial cleaning, gold showed up on one of them... When before one of silver in a lot was reason for celebrating.
Upon further cleaning, and microscopic examination, it turned out to be a gold plated copper.
Just goes to show you that even 1500 years ago counterfeiting fraudsters were active. Still, it was a neat find if not as exciting or valuable as I first hoped.
Those concreted coins that used to sell on eBay etc were supposedly found by metal detectorists in the former southern Warsaw Pact countries, but largely were modern fakes. There is an ancient coin collecting specialty that concentrates on ancient counterfeits though.