Museum officials said on Wednesday that the box was a wedding anniversary gift from a San Francisco man to his wife in the 1800s. It was taken during a break-in on Monday.
It had been in the museum's collection since 1960.
Oakland Mayor Jean Quan's chief of staff, Anne Campbell Washington, told the San Francisco Chronicle (http://bit.ly/WLZwJ0) the box was valued at more than $800,000.
Monday's break-in was the second at the museum in as many months. Pieces of gold and other valuable historic artifacts were taken in November.
Another Northern California museum and a county courthouse have also been hit by gold thieves over the past year.