Copper is a highly reactive metal and binds readily with various compounds in the atmosphere. These reactions are responsible for not only causing the dull brown or pitch black but also the beautiful rainbow tones we often see on copper coins. The colours are so intrinsic of copper coins that grading companies assign special colour designations to these coins as well, namely red, red-brown and brown. It all depends on the amount of red that still remains on the coin. The video concludes with some samples of coins with different colours and the grade received by NGC.
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