For sale, a piece of medieval magic: It's not often that the horn of a unicorn comes up for auction. Andrew Graham-Dixon examines the mysterious past of a fabulous treasure

David Ekserdjian, head of the sculpture department at Christie's, describes it as one of the most extraordinary finds of his career: 'It was wrapped up in newspaper, inside a cardboard tube, but the minute I held it in my hand I knew I was in the presence of a great and extraordinary object. There is something about its weight and heft, as well as the sheer beauty of its carving. It has an almost tangible power, something you can feel coursing through your veins. To hold it is, literally, like holding a piece of history.'

Lot 33 in Christie's forthcoming sale of European Sculpture and Works of Art, 'The Property of a Lady', is described in the somewhat bald language of the auction house as follows: 'A Carved Horn, English, Mid-12th Century.' It is, in fact, one of the great fabled treasures of the Middle Ages - the horn of a unicorn.

Lot 33 is, indeed, a piece of history. It is also a fake - although its fakeness is, unusually, a vital part of its authenticity, since it is not modern fakeness but medieval fakeness.

The Christie's horn is not, of course, to be scientifically precise, the horn of a unicorn. It is the horn of a narwhal, or Monodon monceros, a small Arctic whale - still occasionally sighted today - with a single horn that can grow in the adult male to a length of 9ft. Such trophies, obtained on very rare occasions during the Middle Ages, presumably by brave and hardy whalers working in freezing seas, were passed off and accepted as the horns of unicorns.

The Christie's horn is not large, just 4ft long and only a little thicker than a snooker cue, but it contains a whole world of long-dead beliefs and superstitions. While medieval narwhal hunters maintained a conspiracy of prosperous silence on the subject, belief in the existence of unicorns flourished. There was even a generally accepted (if largely untried) method of catching them. According to Guillaume of Normandy, a 13th-century French cleric: 'Hunters can trap the unicorn only by placing a young virgin in his haunts. No sooner does he see the damsel than he runs towards her, lies down at her feet and places his head in her lap.' Guillaume would probably have been disappointed to learn the more prosaic truth and perhaps, in deference to a more charmed and charming past, narwhal horns should still be referred to by their older and more fanciful name.

The enormous rarity value of unicorn horns was further enhanced by the miraculous properties thought to inhere in them. It was popularly believed that a unicorn horn, should it come into contact with any form of poison, would change colour dramatically: most useful in an age when murder by poison was a particularly favoured method of assassins. A 14th-century French royal inventory records 'the horn of a unicorn, for the touching of all meat and drink to be served'.

John Dekker, the early 17th-century dramatist, wrote of 'the Unicorne, whose Horne is worth halfe a City'. He was scarcely exaggerating. By the time of the Renaissance, it was the single most precious commodity in the Western world. The English crown jewels, in the time of Elizabeth I, contained 'the horn of a unicorn about eight spans in length, valued at pounds 10,000', approximately pounds 10m in today's money. However much the Christie's horn fetches, it can be said to have considerably depreciated in value.

Unicorn horns have not, for the most part, survived the death of the superstitions which once gave them their value, and only a precious few remain in existence. But what makes the Christie's horn a rarity among rarities is the fact that it has been carved. It is one of only two such objects in existence. The other is to be found, languishing in obscurity, unimaginatively displayed and uninformatively labelled, in the English medieval treasury at the Victoria & Albert Museum. The workmanship of both suggests that they may be a pair. If so, given the extreme scarcity of unicorns, it is probable that both were carved from the single horn (the V & A will doubtless be a major bidder on 5 July).

The carving of the Christie's horn is extraordinarily beautiful. Intricate spiral friezes run up and around it, decorative grotesqueries - dragons chasing dogs, figures peeping through foliage, jesters with belled caps - that recall the subtle and highly animated intricacies of early English stonecarving or manuscript illumination. Its original function, however, remains something of a mystery.

Its carved bands alternate with uncarved, blank bands, pockmarked with holes and broken pinheads, suggesting that these plain surfaces were once covered with thin strips of gold or copper. Halfway along it there is a blank patch the size of a human hand. It was made to be held and may have been a ceremonial candle-holder. It is not hard to imagine it being borne through an English cathedral in one of the spectacular processions that were part of the medieval Catholic church year.

Why, it may reasonably be asked, should a unicorn horn have been regarded as a proper object for use in church ceremony? The origins of the unicorn myth lie buried deep in history, but its appearance in the art of ancient Mesopotamia and ancient Egypt suggests that it was once a symbol or god of male fertility. But its original pagan, and probably phallic, significance was adapted and purified by the early fathers of the Christian church, for whom it became a symbol of God's mystic impregnation of the Virgin Mary. Guillaume of Normandy tells us that, 'The unicorn represents Jesus Christ, who took on Him our nature in the Virgin's womb, was betrayed by the Jews and delivered into the hands of Pontius Pilate. Its one horn signifies the Gospel of Truth.'

This complex of beliefs almost certainly lies behind the Christie's horn, although it may not always have been an object of purely Christian veneration. Scientific tests have yet to be carried out on it, but it is an intriguing possibility that the carving may have taken place considerably later than the catching of the narwhal. Carbon-dating recently showed that the walrus tusk from which another great English medieval object, the 12th-century cross in the Cloisters Museum in New York, was made had been existence for some 500 years before it was carved. This raises the possibility that the tusk itself was considered to be some kind of magically potent object long before it was turned into a specifically Christian object, and it is likewise possible that the Christie's horn was once an object of pagan English worship - a fetish object appropriated by the church, perhaps as part of the process of Christianising an unconverted community.

We can be sure that Lot 33's first owner was a narwhal, but after that all is mystery. A brief note on its provenance in the sale catalogue establishes only that it was 'purchased for pounds 12 among a bundle of walking-sticks by the father of the present owner at the sale of the contents of a house in the cathedral close at Hereford in 1957'. Rumours of its existence have, almost ever since then, circulated among those with a special interest in the antiquities of medieval England, and unconfirmed stories tell of it hanging, dusty and apparently neglected, in the back of a London antique shop. Its owner is said to have refused all offers for it. 'That's the family fortune,' he would reputedly tell would-be purchasers. If such stories are true, he may have died and, finally, bequeathed his treasure to the present owner. Christie's, who have been committed to secrecy by the vendor, can neither confirm nor deny such reports.

The precise identity of its previous owner, whose unfortunate descendants allowed it to be sold off so cheaply, is also a mystery. Living in a cathedral close, he may be assumed to have had connections with the church - but just where he obtained this curious, beautiful, richly mysterious thing will almost certainly never be known.

Where Lot 33 will end up and how much it will be sold for remain, for the moment, equally unclear. No other object like it has ever been sold at auction before. 'Estimate on request', the Christie's catalogue advises, which is an auctioneer's way of saying that if you have to ask, then you can't afford it.

David Ekserdjian believes that 'we are talking hundreds of thousands of pounds, at least,' and he cites the example of a fine but by no means unique Gothic ivory mirror case which Christie's sold last December for pounds 115,000. But even if it sells for pounds 1m or more, it will (as Elizabeth I, among others, would have recognised) be cheap at the price. This richly beautiful, mysterious fragment of the British past should be bought for the nation. If it is not, it will be a national disgrace.

(Photographs omitted)