Once Jennifer Aniston got serious with boyfriend Justin Theroux, it seemed the entire country was counting down the days until they got engaged.

But Aniston said she had absolutely no idea it was coming when Theroux decided to pop the question during a dinner for his 41st birthday.

Theroux proposed to Aniston at the Blue Hill restaurant in New York City's West Village neighborhood in August, and the 43-year-old actress told People Magazine this week that she was completely shocked.

"It was one of the sneakiest jobs I've ever seen. I had no idea," she said of Theroux's proposal.

Aniston also spoke to the magazine about the enormous engagement ring Theroux gave her, which jewelry experts have estimated contains an eight-carat diamond and cost about $500,000. The actress said Theroux picked out the ring completely on his own, and that she thought it was perfect for her taste.

"I didn't know what I wanted for a ring. He just knows what I like," she said.

While many fans have marveled at the enormous stone on Aniston's finger, some jewelers balked at its size and criticized the ring as "gaudy."

"Jennifer's engagement ring is valuable, for sure, but a stone that size is not exactly sophisticated," one jewelry expert said. "In fact, one might argue that it is downright gaudy. It looks like it belongs on the hand of a Real Housewife. I love Jennifer's clean and elegant aesthetic, but this ring is completely at odds with that."

The surprise proposal from Theroux shouldn't have been totally unexpected, as Theroux has a habit of surprising his fiance with extravagant gifts. Earlier this month he splurged on a $15,000 platinum pendant adorned with sapphires from Tiffany & Co.

"Justin thought the necklace was a couple of thousand - and went into shock when the saleswoman told him it was just more than $15,000," a source told The National Enquirer. "He took a big gulp, pondered a minute, then said, 'Oh what the hell!' He told the woman to wrap the bauble, saying he'd save it for a special occasion."