The Oppenheimer Blue, the world's largest blue diamond, is to auction at Christie's.
It could set a new world record when it crosses the block in Geneva on May 18, although it will have to surpass its $38m-45m valuation.
The record for any diamond stands at $48.4m, set last year for a blue diamond named the Blue Moon of Josephine by its new owner.
The Oppenheimer Blue weighs in at 14.62 carats and was named after former owner, the late Sir Philip Oppenheimer - who headed diamond giant De Beers.
Francois Curiel, chairman of Christie's Asia, explains: "The Oppenheimers have been leaders in the diamond industry for generations, and Sir Philip could have had any diamond he wanted, but he chose this one, with its perfect hue, impeccable proportions and fabulous rectangular shape."
Blue diamonds are extremely rare and have led the market in recent years.
Tom Moses, executive vice president at the Gemological Institute of America, told Bloomberg: "Achieving the strongest colors in traditional shapes, such as the 15-carat Oppenheimer Vivid Blue can only be achieved with a highly saturated intrinsic color of the rough diamond.
"This blue diamond's color and clarity, combined with its traditional cutting style and provenance, is truly exceptional."
The lot is not the only major diamond coming up for auction in the next few months. The De Beers Millennium Jewel 4 will sell with an estimate of up to $35m at Sotheby's.
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