Peter Brant Jr. lost a pricey piece of jewelry at the New York Botanical Garden’s Winter Wonderland Ball.

We’re told that Wempe Jewelers — which sponsors the high-end fête — loaned the son of Peter Brant and Stephanie Seymour a $27,600 spider-shaped diamond-and-pearl brooch. But it fell from his lapel.

“Peter noticed the brooch went missing when he was outside smoking,” said a source, adding that Brant began a hunt. “He was very upset.”

Luckily, an eagle-eyed guest found the bauble by the bar!

Also spotted were Georgina Bloomberg, Gillian Hearst, Lili Buffett, Ariana Rockefeller and Wes Gordon.