Why is it that when you scratch grime off an object, it is brown but when left as is, is black?

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Why is it that when you scratch grime off an object, it is brown but when left as is, is black?

In: Chemistry

It may be a matter of oxidation. Oxidation is essentially when oxygen reacts to something, which it really likes to do. Oftentimes scraping at something exposes an inner surface, which has not yet been subjected to the oxygen in the air. This being said, a surface experiencing a colour change is a fairly surefire determination that oxidation has occurred at that point.