Officers responded to the Robbins Brothers jewelry store at Torrance and Hawthorne boulevards when an alarm sounded at 5:30 a.m.
They discovered a window had been smashed, Torrance police Sgt. Steven Jenkinson said. Officers searched the building, but no one was inside.
The burglars broke the window to a room behind the showroom and collected several fake cubic zirconia jewels.
The loss was estimated at $500. The real jewelry was locked away, police said.