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davidcyclebackTarnishing (dark brown, green) is a good sign of authenticity. Modern metal printing on cards doesn't tarnish, and certainly doesn't turn green. If it's a homemade computer reprint, there will be no metal, just the picture of metal. The metal on a T205 shines differently than the rest of the card and you can feel the texture with your finger.
As T205s have a metal dust, the borders have a dusty, speckly surface. If the borders are super smooth, like with a bar of gold, it's modern.
Gilding was the last step in making the cards. Technically, it's considered a type of printing, though is obviously different than the standard ink printing. First, glue was applied to the borders, then the metal dust was sprinkled on, the dust sticking the glue. Again, if dust is missing, the borders behind are light yellow. The borders underneath were probably printed yellow to hide any metal chipping, flaking.