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If was in ground, would it not have dirt or erosion etc?
Yes. But the green oxide would be quite tightly bound to the metal, not peeling off like the paint on my house, leaving bright shiny metal underneath. Genuine patina forming on a copper coin over thousands of years should be much thicker than that.
See
these examples on zeno.ru to get a feel for what genuine patina looks like on dug copper cash coins.
This bright green colour is very typical of cash coins from Vietnam. I've heard tales from folks that have been there that to make fake green patina, they take genuine coins that are too badly corroded to identify, crush them up, and mix the coppery-green powder with glue to paste it onto the coin (or other bronze artefacts they're trying to fake). The net result looks... well... just like this.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis