@norm, if it is the right size and the right weight, then I think it must be the right metal alloy. At least with US clad coins, being buried in acidic soil as mentioned by @bojan can produce a similar dark surface staining.
"If you climb a good tree, you get a push."
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz