The Shere Khan Necklace contains 182 diamonds combining for more than 11 carats, and close to 1kg of 18 carat gold.
Featuring two tigers, it was made especially for the Born Free Foundation which is auctioning the necklace, by Channel Islands jeweller Catherine Best.
The charity seeks to protect threatened species and prevent animal suffering.
The necklace takes its name from the tiger in Rudyard Kipling's much-loved Jungle Book.
Lumley, known to many for her work as Patsy in Absolutely Fabulous and a tireless campaigner for animal welfare, has modelled the necklace for the foundation's publicity shots.
"I feel very much that it's almost a royal piece. The necklace is so special it can be worn with anything, though I feel a strapless evening gown and long evening gloves would show it to perfection."
"It feels great, like a badge of office, but it's incredibly comfortable and easy to wear. It reminds me of the fabulous pieces in the Duchess of Windsor's collection - panthers and leopards. I adore extreme looks and this is blingtastic" she added.
We think that the necklace could comfortably meet its estimate due to the triple whammy of its striking appearance, good cause and high profile, thanks to Lumley's involvement.
Only sealed bids are being accepted for the piece, with the auction set to end on June 17.
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