Chris Pratt proposed to Katherine Schwarzenegger with a gorgeous oval sparkler

Following a whirlwind romance over the summer, Chris Pratt asked Katherine Schwarzenegger to marry him with a stunning diamond sparkler.

Pratt, 39, announced the news via Instagram early Monday morning with a photo of himself embracing Schwarzenegger, 29, who was proudly wearing her huge engagement ring.

Pratt selected a romantic oval center stone, which appears to be set on a very thin simple band, for his bride-to-be. According to jewelry expert Jason Arasheben of Jason Beverly Hills, Schwarzenegger’s ring appears to be a 5-carat diamond, which he says can range anywhere in price from $150,000 to $550,000.

“That is the hottest shape right now,” Arasheben tells PEOPLE. “It seems like 10 years ago no one asked for it and now everyone does.”

“Sweet Katherine, so happy you said yes! I’m thrilled to be marrying you. Proud to live boldly in faith with you. Here we go! 💍🙏♥️,” Pratt captioned his announcement.

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Pratt and Schwarzenegger’s relationship first heated up 10 months after the actor’s separation, when the couple was spotted out together on Father’s Day having a picnic in the park in Santa Barbara. They were later seen on dates around Los Angeles and outings with Pratt’s six-year-old son Jack in tow.

Though the pair, who met through Schwarzenegger’s mother Maria Shriver (her father is actor Arnold), have only been together since the summer, they quickly knew they were meant to be. “They’re both serious about the relationship and hopeful for the future,” a source close to the couple recently told PEOPLE. “Chris doesn’t just date to date, and neither does Katherine. She’s very excited.”

“Chris and Katherine are very happy together,” added the source. “They’re very compatible and have a lot in common.”

The engagement comes two months after Pratt and Faris settled their divorce and each waived their right to spousal support. They’ll continue to co-parent Jack and stipulated they live within a 5-mile radius of each other until their son completed the sixth grade.

They seem to be amicable, even taking their son out trick-or-treating with Schwarzenegger and Faris’ cinematographer boyfriend Michael Barrett in October.

“Along with Katherine and Michael, they all took Jack trick-or-treating in Pacific Palisades,” a source told PEOPLE. “It seems they all get along really well. Everyone had a good time. They were making an effort to make sure Jack had a great Halloween.”