Chrome Tourmaline and Sapphire Ring. Photo courtesy Tiancheng International Auctioneer Limited

Centring on a cushion-shaped chrome tourmaline weighing 4.25 carats, framed by blue sapphire surround within a blue sapphire outer border alternated by trapeze-cut diamonds, to diamond-set bifurcated shank. Mounted in 18 karat yellow gold. Ring size: 6 3/4. Estimate US$13,000 - 20,000

Chrome tourmaline is coloured by chromium and vanadium, this rare tourmaline displays a rich, intense green that rivals the very finest emerald and tsavorite garnet. It was first mined in Tanzania in the 1960's and is not often available. Generally chrome tourmaline can only be found in small sizes, gem quality stones over 2.50 carats are considered very rare.

Accompanied by Tokyo Gem Laboratory report numbered 16060479 dated 19 June 2006, stating that the 4.25 carat tourmaline (Chrome) is natural, of green colour.

Tiancheng International Auctioneer Limited. SPRING AUCTION 2013 JEWELLERY AND JADEITE. 15 Jun · 3 pm. Hong Kong Headquarters 30/F Bank of China Tower 1 Garden Road, Central Hong Kong http://www.tianchengauction.com/