(IDEX Online) - A rare diamond, ruby and emerald medallion, dating back to 1789, is to be auctioned by Christie's London.It's one of several that were commissioned by Britain's Queen Charlotte as a gift to close friends, to mark the recovery of her husband King George III from what was subsequently diagnosed as life-threatening porphyria.The medallion being sold was later converted to a locket, containing the hair of both Queen Charlotte (1744-1818) and King George III (1738-1820).The gold-rimmed red, white and blue medallion has an enameled crown, set with diamonds, rubies and emeralds.It will be sold at Christie's Classic Week in London, June 30 - July 14, with a high estimate of $6,300.Pic courtesy Christie's