The theft happened around 6:53 p.m. on Friday, November 12 at Carroll's Jewelers.
Surveillance video shows the man walking up to the counter. Police say he asked to see specific diamonds.
An employee takes the diamonds out and places them on the counter, but as she does the man grabs the jewels and runs from the store.
Police say the suspect was seen fleeing the area in a dark blue Hyundai Santa Fe.
"Before anyone even realized what happened it was over," said Rob Schnell, the manager of Carroll's Jewelers.
After reviewing the surveillance video, one of the employees realized the man had been in the store days before the heist.
Surveilland video shows diamond theft from Doylestown store
"We realized he had been in a couple of times, and was probably looking for the best way to start and the best way to get out," said Schnell.
The thief nabbed two loose diamonds and an engagement setting.
The store has been a fixture in the town for more than 50 years. Customers were alarmed by the crime.
"You don't think it is going to happen in your area because we live away from the city, but is just getting closer and closer. So it's a shame," said Margie Roach of Wrightstown.
"He had a lot at risk with all theses cameras, and the police in this area - they are real close and they come in here all the time. That was pretty brazen," said Joe Rynkiewicz of Pipersville.
Employees say the suspect was the last would-be customer for the day and likely left his fingers prints all over the display case. They say insurance will cover the loss.
They are just thankful it was only a case of theft.
"No one was hurt. That is the important thing. There was no weapon put in anyone's face, there was no threat. It was a literally a grab and run," Schnell said.
Anyone with information is asked to contact Detective Sergeant Paul Kreuter at 215-345-4143.