China is the overwhelmingly major supplier of rare earth minerals to the rest of the world. (South Africa has a tiny share.)
Rare earth minerals (or elements) are critical factors of production of many high technology items such as super-conductors, catalysts, rare earth magnets and batteries of the kind used in hybrids and laptops.
With that background, you can imagine what a problem it is when China unofficially halts export of rare earth minerals to Japan over a disputed arrest of fisherman. Now, after the US has begun to investigate Chinese trade practices, China has unofficially halted exports of these crucial resources to the US as well.