Jennifer Aniston Flashes Engagement Ring on Hot Date With Justin Theroux

Jennifer Aniston and Justin Theroux on October 17, 2011 in Beverly Hills, California. Lester Cohen/Getty Images For ELLE

They have yet to marry — or even reveal a wedding date — but it appears that Jennifer Aniston and fiancé Justin Theroux are already in the honeymoon phase!

On Saturday, the blissed-out couple had a very romantic night out at the celeb-fave spot Chateau Marmont in Hollywood, where Aniston, 43, and her Wanderlust costar, 41, hung out at a private, tucked-away table in the garden area, an onlooker told Us Weekly.

Casually clad in black top, blazer, jeans and black ankle boots, Aniston wore minimal makeup, with her hair down; the most eye-catching part of her ensemble was her enormous engagement ring. Although Theroux proposed back in August, Aniston didn't publicly debut the rock until last Saturday, Oct. 6 in Santa Fe, New Mexico.

A glowing Aniston gazed lovingly at her hunk (wearing jeans, white t-shirt, jeans and leather bomber jacket) as they chatted during their meal, with Theroux reaching out to rub her hand. Actor-screenwriter Theroux rubbed the small of her back as she rose from the table; as they slowly ambled out of the hotspot, the husband and wife-to-be held hands.

Aniston has been shooting We're the Millers in North Carolina for the past couple months.

Of the star's huge new bling? "It looks like a radiant cut center diamond, approximately 8 carats," jewelry designer Peter Meksian from Michael M. told Us Weekly, estimated that the bauble cost an approximate $500,000.

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