With February such a hot month for proposals, it seemed only fitting to seek out the mother of all engagement rings (sorry, Kate Middleton). Last November, the diamond house Graff acquired what is thought to be the finest pink diamond in the world—a 24.78-carat stunner that remained squirreled away in a private collection and not exhibited on the open market for 60 years.

“It’s the finest pink diamond I have seen in the history of my long career,” notes Laurence Graff, chairman of the London-based house he founded in 1966. “It is so exceptionally rare, so magnificent, I doubt if there will ever be a pink diamond to compare.” The price? Well, if you have to ask...

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