The 27-year-old flashed the jewel as she left a West Hollywood restaurant this week before, according to E! News, quickly concealing her hand from the prying eyes of photographers.
An engagement or just a piece of jewellery on an engagement ring finger? We've all had that experience where the engagement ring finger is the only one the ring actually fits on…
Reps for Johnny and Amber haven't commented on the speculation but that's probably not surprising given how private the couple have been about their relationship.
They met on the set of 2011 movie The Rum Diaries but were not romantically linked until the following year.
Both prefer to keep the focus on their professional lives, but at the weekend they did attend The Art of Elysium HEAVEN Gala together in Los Angeles. Despite arriving separately, they are said to have sat next to each other at dinner and been openly affectionate in front of other guests.
Johnny's last relationship was with Vanessa Paradis, the mother of his kids Lily and Jack. They were together for 14 years before stunning fans with their break-up.
Amber, meanwhile, dated photographer Tasya van Ree for around four years.
Celebrities who have confirmed engagements in 2014:
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