The Oakland Museum of California announced Wednesday a $12,000 reward for the safe recovery of a Gold Rush-era jewel box that was stolen from the museum during a break-in on Monday morning.

The object was described as a 7-by-9-inch quartz- and gold-encrusted box that weighs about 3 pounds and depicts scenes of early pioneer life.

Museum director Lori Fogarty said the box was a wedding anniversary gift for the wife of a California pioneer who had come out West in the late 1800s.

"This truly is an invaluable item in terms of California history," Fogarty said.

She said the object had last been appraised about 30 years ago, and that she did not know its current worth. The box has been part of the museum's collection since the 1960s, she said.

Another item was taken in the Monday burglary, but it was of less historical and monetary value, Fogarty said.

The burglar was caught on surveillance video, and was described by police Wednesday morning as a black male with a medium complexion who is 5 feet 9 inches to 6 feet tall. At the time of the burglary, he was wearing a white mask, a dark hat, dark jacket and dark shoes.

Police were investigating whether Monday's burglary is connected to an overnight burglary at the museum that was discovered the morning of Nov. 13. In that case, several items, including gold nuggets, were taken from display cases.

"There were striking similarities between the two incidents," Oakland police Lt. Oliver Cunningham said at a news conference at the museum Wednesday morning.

In both burglaries, the suspect forced entry in a similar manner and ran off with items.

There are now three empty spaces where display cases in the Gold Rush section of the California history exhibit were burglarized. In front of one of the empty spaces there is a sign that reads, "Artifact temporarily removed."

The gold-and-quartz box was taken from an alarmed Plexiglas case, police said.

Because the museum is near the Oakland estuary, police had a marine unit searching the waterway for any evidence, Cunningham said.

Authorities were concerned that the stolen jewel box could be melted down for its materials or otherwise destroyed.

"This isn't something that can just be sold on a street corner," Oakland Mayor Jean Quan said that morning. "This is a theft not just of a valuable item, but of our history."

Fogarty said the museum is under 24-hour security and, since the November incident, the museum has added cameras and guards.

Security guards at the museum are not armed, and in the case of a forced entry are required to call Oakland police, who responded Monday within minutes, according to Fogarty.

Cunningham said it was possible the suspect may have inside information about the museum or has been monitoring the space.

The museum was closed Monday and Tuesday. It reopened as scheduled Wednesday at 11 a.m.

Anyone with information on the burglaries was asked to call the Police Department's Major Crimes Section at (510) 238-3951 or the tip line at (510) 777-2805.

Information could also be given through the Oakland Museum of California website at museumca.org/reward.