Jessica Simpson showed off her brand new Neil Lane engagement ring at Dillard's department store in Kansas City on Saturday as she was promoting her clothing line.

Her boyfriend of five months, Eric Johnson, proposed to Simpson with a three-stone ring that features a central ruby flanked by two diamonds.

"She was in a really good mood," an eyewitness at the store tells PEOPLE. "She was very friendly and sweet. She signed autographs and seemed to have a great time."