The 'Crazy in Love' singer - who has been dating record executive and rapper Jay-Z since 2002 - was with her beau at New York's Butter nightclub when fans spotted her wearing the diamond sparkler on her ring finger.
One eyewitness said: "She was wearing a huge diamond ring on her ring finger. When she noticed that I and some others noticed the rock, she quickly hid her hand.
"Then she reached behind her to where her bag was, and after placing her hand in her bag, the ring disappeared!"
Meanwhile, the former Destiny's Child singer has signed up to star in new thriller 'Obsessed', in which she plays the wife of a man targeted by a female stalker.
She will stars alongside 'Heroes' beauty Ali Larter in the film directed by Steve Shill.
This will not be Beyoncé's first foray onto the big screen. She received critical acclaim for her role in 2006 hit movie 'Dreamgirls' and recently announced she will be playing blues singer Etta James in 'Cadillac Records'.
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