Johnny Depp and Amber Heard
Johnny Depp and Amber Heard are rumoured to be engaged. Reuters

Johnny Depp and Amber Heard are said to be secretly engaged.

The Rum Diaries actress was seen wearing a huge diamond ring as she headed to dinner with a friend at West Hollywood's Ago Restaurant on Tuesday night.

Heard attempted to conceal the ring on her wedding ring, fuelling speculation of an engagement between her and The Lone Ranger star.

According to a source, the couple have been secretly engaged for "a while."

A source told Us Weekly: "The proposal happened a while ago. She just didn't wear the ring."

However in a conflicting report Just Jared stated: "The ring is a promise ring Johnny gave Amber as a gift over the holidays."

The ring is described as a large single emerald or princess cut stone in an elaborate platinum setting.

Depp, 50, and Heard first met on the set of 2010 film The Rum Diaries. The 27-year-old joined the actor on his promotional tour for The Lone Ranger last year, but the couple have otherwise maintained a low profile.

Johnny Depp and Vanessa Paradis
Johnny Depp and Vanessa Paradis split in June after fourteen years. Reuters

The pair made their first official public appearance last week at the Art of Elysium's Heaven Gala in Los Angeles, where they happily posed together for photographers.

Depp began dating soon after he announced his split from partner of 14 years and the mother of his children, singer and actress Vanessa Paradis, in June 2012.

The former couple of 14 years remain devoted co-parents to daughter Lily-Rose, 14, and son Jack, 11, who live with their mother.

The actor famously dated Winona Ryder and fashion icon Kate Moss before his marriage to Paradis.