ON THE BLOCK: One of the world’s rarest pearl necklaces will return to the auction circuit this October. Sotheby’s will auction “The Cowdray Pearls” at its Oct. 7 Hong Kong Magnificent Jewels sale and estimates the Cartier-signed piece could fetch between $4.5 million and $7 million.

The necklace itself is composed of 42 extremely rare gray saltwater pearls, and the auction firm says it is a near-impossible occurrence to find that many gray pearls that create a proper match. The necklace is accompanied by a pair of matching earrings, with mounts also by Cartier.

Chin Yeow Quek, deputy chairman of Sotheby’s Asia and chairman of international jewelry, Asia, said of the lot: “Natural saltwater gray pearls are rarely seen at auction and the present necklace, strung with 42 superb gray pearls and of aristocratic provenance, is arguably the greatest of its kind in existence. This is an extraordinary collecting opportunity for pearl and jewelery connoisseurs around the world.”

Sotheby’s first auctioned the necklace in 1937. The piece originally belonged to the Viscountess Cowdray, Lady Pearson, who died in 1932.

The auction firm also holds the record for selling the most expensive natural gray pearl at auction, with the December 2012 sale of the Wrightsman Pearl brooch for $1.9 million at Sotheby’s New York.