HONG KONG (Reuters) - An extremely rare, vivid purple-pink diamond mined in Russia is expected to fetch up to $38 million when it goes under the hammer on Nov. 11, the auction house Sotheby’s said on Monday. The oval gem, which is named after a Russian ballet ‘The Spirit of the Rose’, is the largest of its kind to be offered at auction. The trend for coloured stones has increased as an asset class by the super rich in recent years. Mined by Russian diamond producer Alrosa, the 14.83-carat diamond was cut from the largest pink crystal ever found in Russia, Sotheby’s said. “The occurrence of pink diamonds in nature is extremely rare in any size. Only one percent of all pink diamonds are larger than 10-carats,” said Gary Schuler, worldwide chairman of Sotheby’s jewellery division. Naturally coloured diamonds occur because they posses a particular lattice structure that refracts light to produce coloured, rather than white, stones. Pink diamonds are both rare and aesthetically highly prized by collectors, analysts say. The gem is being shown in Hong Kong, Singapore and Taipei before being auctioned in Geneva on Nov. 11.