The Congo River, Democratic Republic of Congo
Apparently the metal tantalum makes great capacitors for compact hi-tech items like mobile phones and laptop computers. I think its also known as tin-ore, cassiterite or coltan. There was a tantalum rush in Africa in the early part of this century. The price of tantalum was very high. One kilogram cost over a hundred dollars. You could sell a handful for around twenty dollars.
This is a link to an article by Greg Queyranne, a Canadian researcher focusing on conflicts in Central Africa. Thanks to Adbusters.
If you want to know more, you can watch this video.