The 26-year-old actor is said to have splashed the cash on a "beautiful emerald ring" for Stewart and the couple are planning to wed later this year, reported Showbizspy.
"It's a beautiful emerald ring. He's a real romantic and loves to take an opportunity to remind her that she's the one. He gave her a ring because he adores her and wants to marry her.
"They want their wedding to be authentic, with soul. It's important for him that music be a big part of the wedding. He's writing a song for her and plans to perform it on the vintage Fender guitar she bought him for Christmas last year," a source said.