The 44-year-old actor wanted the ring to be special, and worked with Daniel Arik of Happy Jewelers to create a one-of-a-kind piece.
"He did have an idea of what he wanted," Arik tells ABC News. "We showed him a few different pieces and custom made his ring."
Wahlberg chose a 10-carat yellow sapphire for the ring's centerpiece and surrounded the stone with nearly two carats of diamonds. The ring's setting is white gold.
Arik says that the actor "went crazy" when he saw the finished product. "It was really nice. When he saw it, he was stunned."
"The experience was amazing," the jeweler continues. "Out of all the people I deal with, he was one of the easiest, most pleasant people. It was really fun."
McCarthy announced the couple's engagement on her television talk show, The View, on Wednesday. The couple have not yet set a date for the wedding.