OAKLAND, Calif.—Officials say what they're describing as a "valuable artifact" has been stolen in a break-in from the Oakland Museum of California.

Museum spokeswoman Kelly Koski told The Oakland Tribune ( http://bit.ly/TK8cCe) that a California Gold Rush era quartz and gold box with ornamentation depicting early California was taken from a museum display Monday morning.

Koski did not disclose the worth of the box, and would not say if other items were stolen.

Authorities did not provide details about how the museum—which is closed on Mondays—was broken into.

The museum was also broken into in November, with other valuable historic artifacts, including pieces of gold, being stolen.

Museum officials, Oakland police and Mayor Jean Quan will be discussing the most recent break-in at a press conference at the museum set for 9:30 a.m. Wednesday.

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Information from: The Oakland Tribune, http://www.oaklandtribune.com