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For the study, Mathur, an associate professor of geology at Juniata College in Pennsylvania, collaborated with a museum director, another geologist, a coin dealer and other experts to analyze U.S. currency coined from 1828 to 1972.

They determined that while copper in U.S. coins does not carry a distinct signature, copper isotopes do vary depending on the temperature at the time of mineralization. This allowed them to trace metal sources back to their original ore deposits.

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Historical records reveal that from 1800 to 1837, entrepreneur Matthew Robinson Boulton, who was director of the Soho Mint in Cornwall, England, supplied copper for the U.S. Mint. Mathur and his team, however, could see that British copper remained in American cents until 1849, when the supply came from a Michigan mine.

"These Michigan copper deposits fueled our ability to grow as a country," Mathur said. "So much so that in 1865, the director of the mint stated that we should no longer deplete the precious resources in Michigan, and stated we would start buying copper from other locations. I can see this shift in the isotope ratios of the pennies."

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"Things changed with regard to the amount of copper in the early 1900s to 95 percent copper and 5 percent nickel," he said, adding that in 1981, "we changed to 5 percent copper and 95 percent zinc."

The exception to the "older is more valuable" rule is the 1942 penny. Due to wartime needs, it was made out of iron and not copper, according to Mathur.

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