This looks like one for the books.

On May 15, Christie’s Geneva will auction off a 101.73 ct. pear-shape D-flawless, the largest D-flawless diamond ever put up for sale, as well as one of the largest pear-shapes known to exist.

The diamond—which comes with a Gemological Institute of America report—is also an ultra-rare Type IIa, which makes up less than 2 percent of the world’s diamond production.

The stone is expected to fetch $20 million–$30 million, says François Curiel, the auction house’s international head of jewelry.

“This is the largest D-flawless ever to appear for sale at auction and also the most perfect drop to come up on the auction block,” he says. “We expect tremendous interest, not just from high-profile members of the trade but from top privates collectors too.”

The diamond comes from a 236 ct. rough that was found at the De Beers–owned Jwaneng mine in Botswana. It took 21 months to polish, Christie’s said.

Christie’s declined to identify the seller of the stone. De Beers says the stone came from one of its sightholders, but declined to identify the company.

Christie’s release includes a list of world record prices for diamonds at auction, and it clearly expects the stone to join that pantheon.

The Archduke Joseph Diamond set the current auction record for a colorless diamond when it sold for $21 million at Christie’s Geneva in November 2012.

While this gem is by any standard an amazing diamond, it is not the largest D-flawless known to exist. It is smaller than the 273.5 ct. De Beers Centenary, owned by De Beers, as well as the 203.4 ct. pear-shape Millennium Star, which is believed to be owned by Steinmetz.

JCK