WASHINGTON — That quarter jingling in your pocket could be worth a lot more than 25 cents.

An unknown number of Wisconsin quarters that went into circulation late last year as part of the 10-year, 50-state quarter program contain flaws, sparking a frenzy among coin collectors and other treasure hunters that has not been seen for many years.

The quarters, which appear to have an extra leaf on the left side of an ear of corn, are being sold on eBay and in coin shops for hundreds of dollars. In some of the coins, the leaf is tilted up; in others, it is tilted down.

Old Pueblo Coin in Tucson, where the quarters were first discovered, has sold about 10 sets of three quarters — two flawed and one regular — that are graded in mint condition for $1,099 each. Lower-grade sets, which include small dings that regularly crop up in the factory and handling processes, are selling for $300 to $600 each.

"It's really been a crazy market," Old Pueblo Coin manager Ben Weinstein says.

Adding to the buzz is speculation that the coins were created intentionally. The U.S. Mint, which produces the nation's coins, says it is investigating and does not have any information about whether the extra leaves could have been created on purpose. "The United States Mint is looking into the matter to determine possible causes in the manufacturing process," spokeswoman Becky Bailey says.

It is unclear how many of the flawed quarters, which were made at the Mint's Denver operation, were produced. Collectors estimate approximately 1,000 have turned up, mostly in Tucson and a few in San Antonio. While more are expected to be found, the numbers are still likely to be small compared with the 453.2 million Wisconsin state quarters produced.

"We haven't had anything that comes to my mind that has excited collectors quite as much since 1995," says Eric von Klinger, staff writer at Coin World, a publication for collectors. Ten years ago, some pennies turned up with double imprints of the word "Liberty."

Since then, the Mint has improved technology and tightened its manufacturing operations, von Klinger says. That has raised speculation that the extra leaves could have been intentional. Also raising eyebrows is the apparently accurate placement and design of the extra leaves. Plus, the leaves are so clear that they can be seen with the naked eye, suggesting any changes to the die were more than mere scratches.

Fred Weinberg, a coin dealer in Encino, Calif., and co-author of "The Error Coin Encyclopedia," says enough questions have been raised that an intentional change can't be ruled out. Still, he says, "It's hard to believe that somebody would jeopardize their job to do that."