July 20, 2016 09:05 AM

Pippa Middleton can’t stop smiling – and for good reason!

The 32-year-old was spotted out and about in London on Wednesday, and couldn’t help but wear a grin as she walked around with her engagement ring on full display.

The newly-engaged royal sister sported a flower-printed dress with tan wedges during the outing and let her long brunette locks tumble loose over her shoulders. But all eyes were on the huge sparkler on her left hand.

Pippa Middleton

Set in white gold or platinum with a center stone at least three carats in size, “the Art Deco design makes me think it could be bespoke or it could be vintage,” David Christopher Allen of 77 Diamonds tells PEOPLE.

He adds that from the pictures, the sheer size of the octagonal halo of diamonds and the center stone make it a very expensive piece possibly costing more than $260,000.

Peter Jordan/The Sun/News Syndication

“It is an Asscher-cut diamond, and in order to get a beautiful cut the diamond needs to be of top clarity,” says Allen. “If it is a flawless stone then it could be 200,000 (about $263,000).”

Middleton, younger sister of Princess Kate, officially announced her engagement to longtime boyfriend, financier James Matthews, on Tuesday.

James Matthews

Matthews, a 40-year-old hedge fund manager, was also spotted out in London on Wednesday, and sported a sweet smile as he walked a dog. He was dressed casually, wearing a blue T-shirt, shorts and sneakers.

Matthews popped the question on Sunday during a romantic getaway in northwest England’s Lake District. The couple confirmed the news later in a statement.

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“Miss Pippa Middleton and Mr. James Matthews are delighted to announce they became engaged on Sunday, July 17 and plan to marry next year,” the pair said in the statement.

Middleton and Matthews, who originally dated back in 2012, reconnected after Middleton split from her former beau Nico Jackson in October, after three years of dating.

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