The robbery happened around 9:30 Thursday night at the Gold Buyers jewelry store at 2065 Sunrise Mall in Massapequa.
The gunman, wearing a gray hooded sweatshirt and dark jeans, demanded cash and then forced the clerk into a back room and tied him up.
The clerk suffered minor injuries to his wrist.
The suspect fled in an unknown direction with an unknown amount of jewelry and cash.
He is described as a black man in his 20's, approximately 6'10" tall with a muscular build.
Detectives ask anyone with information regarding this crime to contact Crime Stoppers at 1-800-244-TIPS. All callers will remain anonymous.