NEW YORK, July 30 -- Samsung Electronics fell more than 15 percent on Wednesday. Not because of a missed earnings quarter, not because of a product recall - but because of a single unverified report circulating in South Korean financial media that a Chinese state-backed firm had begun mass production of domestic semiconductor manufacturing equipment. That equipment category - the lithography machines and deposition tools that Western suppliers led by ASML, Canon, and Nikon have monopolized - was the one manufacturing advantage that export controls were built around. The market decided, in a matter of hours, that the monopoly may be ending.

The Philadelphia Semiconductor Index, the standard US benchmark for chip stocks, closed Wednesday i...