Police say the suspect entered the More and More Antiques store at 378 Amsterdam Avenue and fled with an undetermined amount of jewelry.
Several employees chased the suspect but he ran into the subway station at 72nd Street and disappeared.
Police found the suspect's backpack inside the subway station but nothing was inside.
"Apparently a guy came in with a gun, stole a bunch of jewelry posing as a construction worker, and ran down the street," said one eyewitness.
Police will be examining surveillance video from inside the store to try and track down the suspect.
