Since the photos appeared, however, it has emerged that the ring is from the Bulgari Serpenti collection, making it unlikely that Josh bought it as an engagement ring.
Fans have speculated that although the 18-karat pink gold ring, complete with 2.51-carat diamonds is something to be treasured, Dawson's Creek star Josh would probably buy a bespoke ring for Diane if he was to pop the question.
Long-term couple Diane and Josh, who have been dating for seven years, have previously denied that they are engaged and have said that they have no plans to tie the knot.
German-born Diane was previously seen wearing a gold band on her ring finger, prompting Josh to clear up any confusion.
"No, we're not engaged," he told ET Canada. "I'm not sure in the modern world that that's the track that we're taking."
Diane, who was married to French actor Guillaume Canet between 2001 and 2006, has also said that marrying again in not on her agenda.
"Without sounding pessimistic, I learned that I don't believe in marriage," she said in an interview with Glamour. "I believe in a commitment that you make in your heart.
"There's no paper that will make you stay," she added. "A guy friend of mine said, and it made a lot of sense, that people should get married at the end of the road, not the beginning."