Diamonds represents an idiom regarding how people can be like diamonds in the rough. Diamonds is also an analogy or a reference to the Bible saying that people can be like gold or precious metals inside fire. The metaphors says that as metal is melting in hot fire, then anything impure will melt off. You use fire to filter metals or to separate metals from other things.

If our hearts were like caves, then we can go inside and pound at the caves to find the diamonds. We can see that diamonds are like treasures. There are mountains full of diamonds all over Africa. In other words, diamonds are not as rare as they say. People guard these mountains and hills.