A rare "fancy vivid blue" diamond ring goes on display in London this week ahead of a November auction where it is seen fetching up to $25 million.

The stone, named "The Sky Blue Diamond", weighs 8.01 carats and is mounted on a ring by jeweler Cartier. It will be sold at Sotheby's Nov. 16 "Magnificent Jewels and Noble Jewels" in Geneva, with a pre-sale estimate of $15 million-$25 million.

"It really is the color of a summer sky with this beautiful brilliance and subtlety about the stone," said David Bennett, worldwide chairman of Sotheby's International jeweler division.