Sotheby's expected the piece would sell for maybe one or two million but it sold for much more.
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The buyer wants to remain anonymous.
The pendant is from the late monarch's private collection.
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Sotheby's is calling it one of the most important royal jewelry collections to ever come to auction, with the pearl pendant setting a new auction record for a pearl.
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The collection also included a diamond brooch, which sold for $2.1 million, and a monogram ring with woven strands of Antoinette's hair, which sold for $440,000.
A total of 100 items brought sold for more than $53 million.