JUSTIN THEROUX ducked into BevHills Tiffany’s after a business lunch looking to find a gift for fiancee JENNIFER ANISTON and immediately fell for a beautiful platinum heart pendant covered with sparkling sapphires.

“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,” said my inside source.

“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.”

NOTE TO LUCKY JEN: If you happen to spot a tiny turquoise bag tucked under the briefs in Justin’s underwear drawer, don’t open it – and don’t get crazy.

It’s NOT for another woman…it’s for you, lucky girl!