In the ongoing contest for control over global supply chains, China has always had one strong card in its deck: Its dominance over the supply of rare earths and other raw materials vital to several advanced technologies. This week, it decided to play that card. The Ministry of Commerce on Monday said it would impose export controls on two little-known metals, gallium and germanium, effective August 1. The move sent shares of Chinese rare-earth miners rising, with investors expecting pric