The vivid blue diamond could sell for 50 million USD at Christie's auction. (Photo: Reuters)

Known as "Bleu Royal," this vivid blue diamond - which is set in a ring - is among the rarest ever to be unearthed."

What makes Bleu Royal so rare and special is its size. At 17.6 carats, it's the largest of its kind," said Rahul Kadakia, Christie's international head of jewellery.

In 2016, Christie's sold a rare, 14.6-carat blue diamond known as the "Oppenheimer Blue" for more than 57 million USD.