Why Africa is rich?

most parts of Africa are poor but some parts of Africa have a lot of money.
Most people, when they think of Africa they think of poor people with no money, but not all of Africa is like that.
A good friend of mine who comes from a part of Africa says that he has a wealthy family there.
they became rich because of their currency, Money in England is worth a lot more over Africa so im guessing my friends parents are from England and lived in Africa and became rich that way.
But ONLY some people in Africa are rich, not everyone.