Oil & petroleum Africa has a lot of oil and what I would do with it is hide most of it away and only sell some for very high costs because to other continents they would think we are running out. Then after a long time when all the other countries have run out of it, I would say we found a huge supply of oil and sell it for a large amount again and basically save the day.
Diamonds I would train a lot of people to be professional jewelers. That way instead of selling all the diamond to other continents we could only make them available in Africa and so other places would have to come to Africa to get any diamonds. And since the jewels would already be cut they would be worth more but not too the jewelers.
Gold I would make the gold go in with the diamonds and have a lot of very valuable jewelry that didn't cost as much to buy since it is found in the area.
Coffee I would only grow the best coffee sows could sell it to Starbucks. Since everyone likes Starbucks we would get a very good name for our coffee and have more people wanting to buy it.