OAKLAND (AP).- The Oakland Museum of California offered a $12,000 reward Wednesday for the safe recovery of a stolen gold-encrusted jewel box — the latest in a series of thefts involving Gold Rush-era artifacts across the region.

The box stolen Monday depicts images of early California history and was originally a wedding anniversary gift from a San Francisco pioneer to his wife in the 1800s, museum director Lori Fogarty said.

It's the size of a small shoebox and weighs about three pounds.

Oakland city officials have said the box was valued at more than $800,000, but Fogarty said it was difficult to put a price on the artifact.

"It's very difficult to assign value to something like this," she said. "But I can say it's a treasure of our collection and a critical piece in our holdings."

It was the second major theft in as many months from the popular Gallery of California History exhibit at the Oakland museum. Gold nuggets and other historic artifacts were taken in November. Police believe the same culprit may have committed both thefts.

Fogarty said the high price of gold — which was selling Wednesday at about $1,657 an ounce — might have prompted the break-ins.

"We don't know if it's related to the value of gold, if that is what the burglar is interested in," Fogarty said. "But that's certainly one of our fears."

Nonetheless, the precious metal appears to have made targets of sites that house historical items. In September, a state mining and mineral museum in the Sierra foothills in Mariposa was robbed of an estimated $1.3 million in gold, precious gems and artifacts by thieves armed with pickaxes.

In February, thieves smashed a lobby display case at the Siskiyou County courthouse and made off with large chunks of gold. Both sites are in California's Gold Country, where people from around the world came in the mid-1800s to strike it rich.

Four people have been arrested and charged in the Mariposa case.

Gold Rush memorabilia and items remain attractive to collectors because of the romance and myth associated with the era, said Gary Kurutz, a special collections curator at the California State Library in Sacramento.

"There are so many great stories associated with it — the letters, the diaries, the rare books, the prints and of course the actual gold nuggets," he said.

Gold can be melted down, making it easy to fence. But Kurutz said someone would have a harder time offloading a jewel box like the one taken on Monday.

"You would not be able to take it to an auction house or any responsible antique dealer," he said. "They would know immediately that this is hot."

Elsewhere in the nation, about $400,000 in gold pieces were stolen from a museum in New Jersey two years ago, and a gold bar worth about $550,000 was taken from a Florida museum in 2010. That gold had been recovered from the shipwreck of a Spanish galleon off the Florida Keys.

Steve Keller, chair of the American Alliance of Museums security committee, said smaller museums that have gold on display are vulnerable to theft, although he did not include the Oakland museum in that category.

The museums "have sort of never had a problem in the past, so they assume they're not going to have a problem now," said Keller, who has his own security consulting company. "But times have changed."

On Wednesday, Oakland Mayor Jean Quan pleaded publicly for the safe return of the jewel box stolen two days earlier. She said the artifact belongs to the city.

"This is not something you can sell on a street corner," Quan said. "We hope those who will be approached will return it to the people of Oakland. This is a theft not only of a valuable object, but a theft of our history."

It was not the first time the jewel box, which has been associated with the museum since the 1960s, was stolen. Fogarty said it was taken in 1978 and recovered at an auction house several years later.

Police declined to go into specifics about the latest theft, citing the ongoing investigation. Fogarty said the thefts at the museum involved items kept just a few feet apart, and the thief apparently broke in through an entrance near the museum garden.

The jewel box had been in a Plexi-glas case equipped with an alarm that sounded after it was stolen, she said, adding that police arrived in less than five minutes.

Both thefts were caught on security cameras, she added.

After the first theft in November, Fogarty said the museum added extra security guards and cameras.

"Obviously, we'd like our security measures to be better or this wouldn't have happened," Fogarty said. "But we do feel confident with that steps we took were appropriate and the enhanced security measures will aid in this investigation."

___

Thanawala reported from San Francisco.