A giant gem will hit the auction block in New York this autumn with the sale of the 32-carat emerald-cut Annenberg diamond, which is expected to fetch as much at $5 million, Christie's said on Wednesday.

The flawless ring-mounted diamond, owned by philanthropist Lee Annenberg, widow of publishing magnate Walter Annenberg, leads the auction house's magnificent jewels sale on October 21.

"Gems of this size are extremely rare, and their presence on the market is always an important event in the world of diamonds," said Rahul Kadakia, Christie's head of jewelry.

"This gem's impeccable color, clarity, and polish as well as its prestigious provenance will attract jewelry collectors from all over the world."

Annenberg, who died in March aged 91, served as chairman and president of the Annenberg Foundation after the death of her husband, a one-time U.S. ambassador to Britain who started the foundation to fund nonprofits, education and the arts.

Annenberg also served a chief of protocol during the first term of President Ronald Reagan.

The gem will go on tour ahead of the sale with stops in Geneva, London, Hong Kong, Los Angeles and New York.

In December the 35.6-carat Wittelsbach blue diamond smashed expectations and sold for $24.3 million, which experts said was consistent with rare and colossal diamonds fetching especially strong prices during times of economic woe.