A rare round blue diamond will go under Sotheby’s hammer in Hong Kong this October, with experts saying it will easily fetch a record-breaking US$19 million, or $2.5 million per carat.

The 7.59-carat gem, named "The Premier Blue," has been graded internally flawless (IF) and brilliant-cut by the Gemological Institute of America (GIA), according to the auction house.

Sotheby's added the rock, which is about the size of a shirt button, is the largest fancy vivid blue diamond ever graded by a laboratory. Its cut is rarely used in coloured diamonds due to the high level of waste involved in the cutting process, even though it is widely applied to white diamonds to maximize brightness. Blue diamonds rarely go on sale and have been sought-after by royals and celebrities for centuries.