It is widely acknowledged that an emerald boasting the quintessential green—the color upon which all other emeralds are judged—can only have come from one place: Muzo, the ­largest and most prestigious of Colombia's mines, located about 60 miles north of Bogotá. But to have indisputable proof that a stone did, indeed, originate there? "Almost unprecedented," says David Bennett, chairman of the Sotheby's Jewellery Department in Europe and the Middle East. That's one reason the superb 12.01 ct. emerald at the center of this ring fetched a staggering $119,000 per carat at Sotheby's Geneva Magnificent Jewels sale on Nov. 15. Consigned to the house by the owners of the Muzo mine, the gem came with an unusually precise provenance. "We even know what part of the mine," Bennett says. But origin alone doesn't account for Lot 467's clobbering of its high pre-sale estimate of $600,000. That d