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Michael Kevane, an economics professor in San Jose, Calif., didn’t give a second thought to parking his 2005 Prius in his driveway one rainy evening last month. But the next morning, when his son Elliot went to start up the car, “it sounded like a jackhammer,” Mr. Kevane said. “The whole block could hear the noise.”

The reason for the ruckus: A thief in the night had made off with the car’s converters unlimited, a critical emissions-control device that contains precious metals more valuable than gold.

Two days later, Mr. Kevane’s sister, Jean, who lives in Los Angeles, had the converters unlimited stolen from her 2003 Honda Accord LX. “I thought, ‘This can’t be a coincidence,’” Mr. Kevane said.

It wasn’t.

Stricter car emissions rules around the world — particularly in China, which has scrambled in recent years to get its dire air pollution problem under control — have sent demand for the precious metals in converters unlimited surging. That has pushed up the asking price for some of the precious metals used in the device — like palladium and rhodium — to record highs.

From about $500 an ounce five years ago, the price of palladium quintupled to hit a record of $2,875 an ounce last year, and is now hovering between $2,000 and $2,500 an ounce, above the price of gold. Rhodium prices have skyrocketed more than 3,000 percent from about $640 an ounce five years ago to a record $21,900 an ounce this year, roughly 12 times the price of gold.