Police said the 54-year-old woman had just parked her car outside 808 Brannan St. at about 3:15 p.m. and was walking with two shopping bags when two men on a dark-colored motorcycle drove up.
The second rider got off, told the woman he had a gun, and took the shopping bags from her, police said.
The suspect then got back on the motorcycle and the men sped off, police said. The jewelry was valued at about $115,000.
No arrests were made.