Two flawless white diamonds each weighing in at more than 50 carats are expected to fetch more than $15 million (10.65 million pounds) when they are auctioned in May, auctioneers Sotheby’s Diamonds said on Friday.

The diamonds, a 51.71 carat round brilliant-cut gemstone and a 50.39 carat oval, are the second-largest of their respective kinds ever to come to auction, Sotheby’s said.

The sale of the gemstones, discovered and purchased at tender in Botswana, comes after a 102 carat white diamond sold to an undisclosed buyer in February far exceeded the price paid per carat for any colourless diamond at auction, according to auctioneers, who did not reveal the price paid.

The record price previously paid was for a 118 carat diamond sold in Hong Kong in 2013, which fetched $260,000 a carat.

Sotheby’s also said it was auctioning “The Farnese Blue,” a roughly 6 carat blue diamond given to the Queen of Spain, Elisabeth Farnese, in 1715, which auctioneers said had been passed down through four European royal families since then.

All three of the diamonds are scheduled to be auctioned in Geneva in May.