Diamonds are forever — but this engagement sure wasn’t.

A Brooklyn man is suing his ex after she refused to return a $40,000, white-gold diamond ring, according to court papers.

Owen Muir handed over the family ring to Stephanie Foley in August 2012. But following their breakup, he wants a judge to force Foley to turn over the ring — or its cash value.

Foley could not be reached for comment.