Unicorns, of course, were mythical beasts credited with magical powers. The smart alecs of the Middle Ages obtained the single tusks of the rare arctic whale, the narwhal, and sold them as coming from unicorns.
This horn had been bought for pounds 12 among a bundle of walking sticks by the father of the vendor at a house sale in 1957. He clearly realised what he was buying since the family has a letter from the Victoria and Albert Museum, dated 1958 - the V & A has the only other carved narwhal horn on record.
Christie's suggests that they were carved in the same English workshop, possibly using a single horn split in two - the diameters seem to match. Christie's horn, which is almost 4ft long, would have been the top end.
Even narwhal horns were tremendously rare. The horn that Pope Clement VII gave the king of France's son as a wedding present in 1533 is said to have cost 17,000 ducats; Michelangelo was only paid 3,000 ducats for painting the Sistine ceiling.
The identity of the buyer is a secret. He or she bid over the telephone, mainly against another telephone bidder.
Meanwhile, at Sotheby's, the National Gallery of Scotland was busily picking up for a song Renaissance medals. The director, Tim Clifford, has been building up a medal collection for some years to complement his Renaissance paintings and sculpture.
For pounds 4,400 he acquired a medal celebrating Girolamo Savonarola (1452- 98), the Dominican reformer who tried to turn Florence into a theocratic state, burning immoral books and pictures - but was eventually hanged and burned himself. Mr Clifford bought seven medals in all; a medal of Ottavio Farnese, second Duke of Parma, cost a mere pounds 396. The National Museum of Scotland also bought three medals.
Sotheby's sale included a particularly fine private collection of Renaissance medals formed by a connoisseur in the 1920s and 1930s. A silver medal designed by Albrecht Durer - the only medal made by the great German artist - set an auction record for any historical medal when it sold for pounds 55,000 to a French dealer. It was made to celebrate the visit of the Emperor Charles V to Nuremberg in 1521, but he never came and most of the medals were melted down.
The sale was very successful, realising a total of pounds 767,000, almost double the presale estimate.