A thief used a hammer to hold up a Boerum Hill jewelry store Wednesday, where he forced workers into a back room before looting the place, cops said.

The man, who was wearing all black, entered the Erica Weiner store at 360 Atlantic Ave. in Boerum Hill around 2:30 p.m. and demanded that four employees and a customer go into the back. He then loaded numerous pieces of jewelry into his backpack before fleeing, cops said.

The value of the jewelry he made off with was unknown, cops said.

The store advertises antique and handmade jewelry for prices that soar into the tens of thousands of dollars.