China is stepping up scrutiny over exports of indium, leading some buyers to fear the niche metal, sought after for next-generation data centers, may be added to the export control regime that has become one of Beijing's most potent trade weapons. China produces nearly 70% of the world's indium, a byproduct of zinc refining mostly used in displays and solder but also the raw material for making indium phosphide, used to make high-speed optical chips for AI data centers.