The British star was a surprise nominee for her role in Mike Leigh's film Secret & Lies, and bosses at Armani lent her jewellery for the glitzy showbiz event.
However, she dropped the pricey piece during a bathroom break and heard some nasty gossip as she crawled around the floor looking for it.
She tells Reader's Digest, "I'd been nominated for Secrets & Lies and had borrowed a hugely expensive diamond brooch from Armani.
"In the loo (bathroom) it popped off and I was scrabbling around on the floor looking for it when I heard two women at the sinks discussing the nominations for best actress.
"One said, 'Well, whoever gets it it certainly won't be Brenda Blethyn!' I was mortified and waited for them to leave before going back to my seat."
Blethyn got her revenge when she won the prize for Best Actress.
She adds, "So it was doubly wonderful when my category came up and I heard the words: 'And the winner is... Brenda Blethyn.'"