The global gold market relies on a network of banks, refineries and couriers that can fly bullion between key trading hubs at a moment’s notice in pursuit of the highest prices. On Friday, a shock US ruling suggesting that the metal would be subject to tariffs plunged that system into chaos.

Some of the most widely used types of gold bars would be subject to U.S. tariffs, customs agency officials ruled, a particular blow to Switzerland’s large gold refining industry.

The U.S. has unexpectedly slapped tariffs on imports of some gold bars, a letter from U.S. Customs and Border Protection showed, roiling the market for the precious metal. Why is this disruptive? The