I have a metal charm on my keychain that used to be very shiny. Due to wear and daily usage on my keychain on some parts it has gotten a deep black color while on some others it looks copper-y, why is that?
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Many metals react with the oxygen in the atmosphere, creating a layer of oxide we call tarnish. This is why copper turns green (the statue of liberty was originally the color of a brand new penny!), why silver turns black, and why iron rusts (rust is iron oxide). One of the main reasons why gold is valuable is not just that it is rare, but also that it doesn’t really react with the atmosphere to create oxides. This makes it an ideal place to store wealth over time.
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