Ashton Kutcher and Mila Kunis
03/04/2014 at 12:00 PM EST
and Ashton Kutcher
have made it official – and she's got the sparkler on her left ring finger
to prove it. But that doesn't mean they'll be heading down the aisle anytime soon.
"It won't necessarily be a short engagement," a friend close to the couple tells PEOPLE. "They haven't begun planning the wedding yet."
Still, they did manage to pull off the unusual feat of keeping a Hollywood engagement under wraps, at least for a little while. According to the source, the duo have already "been engaged for a couple of weeks."
But no one ever said they were the quintessential limelight-loving Hollywood couple – in fact, quite the opposite, and Kunis's ring style is pretty indicative of their low-key romance
"It's a simple band with a round diamond" says the friend of the couple. "No baguettes, no insets. It's not ostentatious, like a Kim Kardashian
ring. She doesn't need a security team with her to wear it. It's a lot like her and Ashton."
Indeed, ever since they began dating in 2012
, they've spent the majority of their time together traveling around the world
or attending various sporting events or family affairs
, instead of going to highly publicized events. At a recent Feb. 19 party in New York for Jim Beam bourbon whiskey, for which Kunis, 30, is the global brand ambassador, the two were very casual and didn't mention the engagement at all.
"They weren't overtly lovey-dovey, but they kissed and you could tell he was proud of her, and she was happy he was there," says an onlooker.
The couple can credit their longtime friendship (they first met 16 years ago, on the set of That '70s Show
in 1998) with their drama-free ride, but a business source of 36-year-old Kutcher's also adds that he adores Kunis's independent streak and sense of adventure.
"He likes her spunk and willingness to experiment with things he likes to do," the source says. "They have a lot in common. Ashton is very much in love."
For much more on their engagement, pick up this week's issue of PEOPLE, on newsstands Friday
Reporting by AILI NAHAS, MARY GREEN and LINDA MARX