The De Beers Cullinan Blue was fashioned from a rough stone unearthed in South Africa last April.

The 15.1 carat gem will be shown off on a world tour – including New York, London, Dubai and Shanghai – before its April sale at Sotheby’s salerooms in Hong Kong.

Patti Wong, chairman of Sotheby’s Asia, said: “Blue diamonds of any kind are rare…but this is the rarest; nothing of remotely similar calibre has appeared at auction in recent years.”

Only five blue diamonds over 10 carat have been auctioned – but none exceeded 15 carat. Sotheby’s said this makes the sale “a landmark event”.