[SVD]

Actually a sulphide on older pieces makes sense - used to be a lot of coal used for heating and power and most coal has at least a little sulphur in it.

Do you see it more on pieces that were worn than on ones that were kept as displays? That would go along with it being environmental exposure to sulphates rather than oxidation (since there'd be just as much oxygen in a display case as out but less exposure to smoke and such).