Over a year into their relationship, Kate Bosworth and Justin Long officially made their red carpet debut at the Vanity Fair Oscars party on 12 March. Wearing a chic plunging gown by Monique Lhuillier, the "Blue Crush" actor smiled for the cameras with her hand on Long's chest, offering a view of her massive diamond ring. Though reps for the actors have not confirmed their engagement, it sure looks like the couple is headed towards the aisle.

Bosworth's ring appears to be a classic, round-cut diamond set on a thin silver band. The sparkler is estimated to be about 10 carats and valued at $500,000, according to diamond expert Mike Fried, CEO of The Diamond Pro. "A diamond of this size is incredibly rare," he told POPSUGAR. Bosworth has yet to share a close-up of the round-cut diamond, known as a traditional shape that never goes out of style.

The new accessory complemented Bosworth's regal gown, which featured a black velvet corset and a floor-length pale pink skirt from the designer's autumn 2023 collection. Coincidentally, Lhuillier is known for her exquisite bridal wear, so perhaps the two are subtly offering a hint at a collaboration in the future. Styled by Samantha McMillen, Bosworth finished the minimalist look with Cicada jewels, including a ring on her right hand and diamond earrings.

Long looked sleek in a black suit and tie, and was seen straightening out his rumoured fiancée's gown while drawing attention to her gorgeous ensemble. The couple appeared blissful as they shared kisses and stared lovingly at one another both on the carpet and inside the party. Get a closer look at the pair's coordinating looks — and the giant diamond ring — in the photos ahead.