Olivier Sarkozy Bought Mary-Kate's Engagement Ring From Sotherby's Auction.

He liked it, so he put a huge vintage Cartier ring on it! Mary-Kate Olsen‘s engagement ring from Olivier Sarkozy has had us buzzing since the moment she debuted it in N.Y.C. Now we have new photos of her showing it off during Paris Fashion Week, as well as more details about the diamond-and-sapphire stunner.

According to a StyleCaster source, the ring was chosen for Olsen at a Feb. 6 Sotheby’s auction, where it sold for $81,250. The Sotheby’s catalog description of the ring lists it as having a 4-carat European-cut center stone, plus 16 calibré-cut sapphires surrounding it and about 1.5 carats of diamonds forming the “petals” around the high-set stone. It dates from 1953 and was previously owned by “a Tennessee collector.”

Olsen, 27, and Sarkozy, 44 -- half-brother of former French president Nicolas Sarkozy -- first stepped out together in 2012. Since then, they've been a frequent presence at basketball games in New York City, where they live together in a $6.25 million townhouse.

This will be the former Full House star's first marriage, and Sarkozy's second. He was previously wed to Charlotte Bernard, with whom he has two kids, Julien, 12, and Margo, 10.