NEW YORK – A square, 32.01-carat emerald-cut diamond that billionaire philanthropist Leonore Annenberg bought for her 90th birthday sold for $7.7 million at auction on Wednesday.

About the size of a walnut, the flawless, colorless diamond sits on a ring designed by Manhattan jeweler David Webb. It is flanked by two pear-shaped diamonds, one of them 1.61 carats and the other 1.51 carats.

The ring was offered for sale by Annenberg's estate. Christie's auction house did not identify the buyer, who bid by phone.

Annenberg died in March at the age of 91. She served as U.S. chief of protocol during President Ronald Reagan's first term — a position that carried the rank of ambassador. Her husband, Walter Annenberg, a billionaire publisher and ambassador to Britain under President Richard Nixon, died in 2002.

The big diamond "combines the best of the four C's: top color, perfect clarity, ideal cut and excellent weight," said Francois Curiel, international head of Christie's jewels.

With the "impeccable provenance of the Annenberg name, you have one of the finest gems to appear on the market for many years," he said.

Annenberg purchased the ring for herself to mark her 90th birthday, Christie's said. It was delivered by armed guards to her Rancho Mirage, Calif., home from the Beverly Hills jeweler's store, it said. She was thrilled whenever someone came by to admire it, the auction house said.

The ring's pre-sale estimate was $3 million to $5 million. The previous auction record for a 30-carat square cut flawless, colorless diamond was $3.1 million, set at Christie's in Geneva in May.

The record for any diamond or jewel at auction is $24.3 million for the 17th century cushion-shaped grayish-blue 35.56 carat Wittelsbach Diamond. It was sold at Christie's in December 2008, topping the previous record of $16.5 million for a 100-carat diamond sold in 1995 in Geneva.

In May, a rare 7.03-carat blue diamond sold at Sotheby's for $9.5 million — the highest price ever for a gem of its kind.

Walter and Leonore Annenberg donated $4.2 billion to cultural, educational and medical institutions through the Annenberg Foundation. In 2002, their collection of French Impressionist art was donated to the Metropolitan Museum of Art in New York, where Leonore Annenberg was a member of the acquisitions committee.

She was also a trustee of the Philadelphia Museum of Art and served on the Trustee's Council of The National Gallery of Art in Washington, D.C.

Her husband established the Annenberg School of Communications at the University of Pennsylvania and at the University of Southern California.