The Insider.com

When Jennifer Aniston debuted her ginormous engagement ring two weeks ago, the diamond's shocking size surprised just about everyone.

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And it turns out, even Jennifer.

"I had no idea, and I had no idea what I wanted. He just knows what I like," she tells People about the insane sparkler. "I was impressed."

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Jennifer is also dishing on the surprise engagement, calling it a "very covert operation."

"[He] worked with the girls that I have worked with for years and years and years, so he kind of was picking the specialists' brains," she says. "He kept it within a very small circle of people. It was one of the sneakiest jobs I've ever seen, actually."

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Theroux proposed on August 10, calling the engagement "an extraordinary [birthday] gift," as the actor-screenwriter turned 41 the same day.