I do think this is how my Roman coin find came to America and ended up on that colonial-era site. The sand roadbed (near one of the earliest churches in GA) was not a ballast spoil area (too far inland), but I think the coin was probably found in ballast or carried over in someone's pocket aboard a ship in the 18th century. Shortages of small change being what they were in the colony at the time, it is not difficult to imagine someone carrying this coin to use as a farthing in change (if not as a curiosity). We relic hunters find an amazing variety of world coinage on early sites here. On one site, within a relatively small area, I found a 1782 British "Hibernia" halfpenny struck for Ireland (actually a contemporary counterfeit), a 1782-A 2-sous piece struck for the French colony of Cayenne (ditto), and a 1779-Mo Spanish Colonial 2-reales coin struck at Mexico City. The two 1782 coins were close enough together that I think they were dropped by the same person at the same time. All of which goes to show you what a mixed bag of stuff was circulating around here at the time.
And then there's the tale of the (allegedly early-15th century) Ming Dynasty Chinese medallion I dug in North Carolina in 1994. But that's another story. (And it wasn't technically a coin.) Gavin Menzies wrote about it in his Who Discovered America book. Menzies' work has always been controversial, but interesting. It was Dr. Siu-Leung Lee who translated (and got excited about) my medallion find, taking it on a lecture tour in Asia (see poster above). I can't tell you anything else for sure except that I did indeed dig it (despite Menzies' critics casting doubt on that). It was on an early site with European settler activity as far back as 1794 and Native American occupation long before that. I have little doubt the artifact was old, and had been in the ground a long time. But 1420s old? Who knows. Prior to Dr. Lee's examination of it I had initially just assumed it was only about 50-75 years old rather than 570.
Yeah, WAY too many people try to claim that coins "prove" something. Coins, before they even got to curios, were metal. As such, they were reused for centuries. Look at Ostrogothic coins. They took old sestertii and made marks on them to use as large denomination bronze. Roman gold coins have been found en mass in Scandinavia and India. Islamic coins from 1000 years ago found in Australia. None of these things meant colonization, it could mean simple trade, shipwrecks, etc. I read about someone claiming they found a Ming "anchor" in SF bay. Maybe the Ming visit, but OTOH the anchor is a round rock with a hole in the middle. It kind of might just be a round rock with a hole in it.