At a press conference Wednesday morning, the Oakland Museum of California announced the theft of a historical quartz and gold-encrusted jewel box from its Gallery of California History. The burglary, which occurred early Monday, is the second the museum has recently experienced; in the early morning hours between November 12 and 13, gold nuggets were stolen from the same collection.

The missing box, which dates back to the Gold Rush era, depicts scenes of early California pioneer life. Its value has not been appraised for at least 30 years, museum director Lori Fogarty said, and museum officials will not disclose its current price. A $12,000 reward is being offered by the museum and its insurance carrier for its recovery, provided that the claimant is not involved with the theft. The reward also includes the return of the gold nuggets, but the museum is currently focusing on recovering the box, Fogarty said.

Similar in size and shape to small shoebox, the artifact measures 7 by 9 inches and weighs roughly 3 pounds. It was given as a wedding anniversary present by a San Francisco pioneer to his wife during the Gold Rush, a period that ran from roughly 1848 to 1860 in which people from around the world moved to the West in search of fortune. According to Fogarty, the jewel box was created by a San Francisco goldsmith and was made with quartz that has not been mined in California since the 1800s. It is a “precious” and “invaluable” component in telling the history of the Gold Rush, Fogarty said, and has been part the museum’s collection since 1960.

Both the Oakland Police Department and the museum will not release a picture of the stolen box until additional investigative leads have been pursued. According to Oakland police Lt. Oliver Cunningham, the first five business days after the theft are the most crucial for investigators to pursue any leads—for reasons that he could not disclose.

“The loss of this is not just a loss to the foundation—the artifact belonged to the people of Oakland,” said Mayor Jean Quan, who spoke at the press conference Wednesday morning along with police officials and Fogarty. “This is not something that you can just—hopefully—go sell on any street corner. We hope that the art community, that people who might be approached for the sale of this artifact, will join with us to make sure that it’s returned to Oakland.”

Cunningham said surveillance footage captured one suspect, whom he described as man between 5’ 9” and 6’ tall, African American with a medium complexion and short black hair. The man was wearing a dark hat, jacket and shoes and a white mask—a detail that Cunningham said is “relatively unique.” The man is believed to have entered the museum forcefully through an exit located near the display and to have fled without being apprehended. Cunningham said the OPD believes only one person is responsible for the crime.

Given the museum’s location, Cunningham said OPD’s Marine Unit isn’t ruling out the possibility that the box was dumped into the nearby estuary. Museum officials are also concerned that it could be melted down and sold for its gold.

Monday’s break-in presents “striking similarities” to the theft the museum experienced in November, Cunningham said, in which gold nuggets were stolen around midnight from an area near where the jewel box was once displayed. The OPD is currently unsure of whether the suspect had any insider information about the exhibit or how to enter the building, Cunningham said. Police believe the person broke into the building using an unknown object.

Security guards were on duty during both break-ins, as the museum operates 24-hour guarded, unarmed surveillance. Fogarty said security guards notified the police of Monday’s theft after viewing the suspect on surveillance cameras and hearing the alarm sound on the Plexiglas case that enclosed the missing jewel box. Assistance arrived at the museum with five minutes, Fogarty said, but it was too late; the suspect had fled.

Added security efforts, including additional guards and several cameras have been put in place since Monday’s invasion. The museum, as well as the Gallery of California History collection, remain open as usual to the public.

Anyone will information regarding either burglary can contact the Oakland Police Department’s Major Crimes Section at (510) 238-3951 or visit museumca.org/reward.