A rare 20.2 carat sapphire known as the Pride of Kashmir is set to auction at Sotheby's Hong Kong.

The gem is mounted on a gold and diamond ring and is expected to make around $3.7m-5.8m when it crosses the block on October 4.

The Pride of Kashmir is an exceptionally pure sapphire

The stone is a deep royal blue and boats an unparalleled purity.

It looks more than capable of beating the $5.1m record set for a 25.8 carat sapphire at Sotheby's last year.

Kashmir has set the gold standard for sapphires since the late 19th century, when a landslip revealed a rich seam of gemstones.

Sotheby's comments: "the sapphires of Kashmir form an exclusive class of their own.

"It is not the location that has earned the Kashmir gem its legendary status, but instead the superiority of their intense soft blue and rarity."

The depth of colour that Kashmir stones exhibit is due to fluid inclusions, which spread light throughout the stone.

However, while beautiful they are very rare - as Sotheby's explains: "…the mining of Kashmir sapphires have only been sporadic and their supply limited due to remoteness and political unrest of the region."

The auction house describes this gem as "the pinnacle of Kashmir's output" and "one of nature's greatest treasures."

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