Quentin Letts: Now, Mr Cameron, remind me what you do...

Mandela's hand has become the most cherished item in the world. Some days it is touched by so many admirers it is left shining

Not since 1905, when a Transvaal mine-worker found he had discovered the Cullinan Diamond, has Africa known such a precious object. It is coveted by the most powerful people in the world. It is held to have near-miraculous powers. Some days it is subjected to the touch of so many admirers that it is left shining like the toecaps of an Aldershot sergeant major.

From the moment Nelson Mandela left prison 16 years ago, his right hand has become the most cherished item in the world. That noble palm has been grasped and pumped by so many people - particularly foreign white politicians - that the only wonder is the wrist above it was not long ago yanked out of its socket.

It has been seized by De Klerk, caressed by Clinton, fondled by Blair, kitten-scratched by Gerri Halliwell and soft-sponged by Gordon Brown's chewed digits.

As with the marble statue of the apostle James at Santiago de Compostela, eroded by centuries of Christian fingers, or the frescoes of Mount Athos, hollowed by the touch of wide-eyed visitors, the Mandela hand attracts travellers from every continent. Its magnetism is mightier than the most superconducting solenoid in Switzerland. Its draw makes our National Lottery seem a very tame bout of bingo.

Last week, David Cameron was the latest to muscle in on the Nelson hand-show. Poor Mandela. Many of us exist in the vague hope that, toward the end of what we hope will be long, fruitful lives, we will be permitted a few placid years pottering around the garden or reading books. Is such a retirement not one of the few consolations that fuels us through this vale of tears; to be able to doze when we want, flick on the wireless and catch up on the cricket without interruption?

South Africa's grandfather is permitted no such rest. Last week, young Cameron, hair slicked back, his beaky nose down like Concorde coming in to land, strode into Mandela's sepulchral presence. Nelson did not stand a chance. One minute the 88-year-old may have been humming the opening bars of "Nkosi Sikelel'i Afrika" and musing on what to do with those proteas in his front flower bed. The next, wham!, his hand was being wrenched off its hinges by an on-the-make Etonian with a moist, red face and a propensity to insert the word "huge" into his every public utterance.

Did Mandela, in that moment, discover how it felt to be the Scottish salmon, lifted from its brackish Highland stream by the claws of a clamouring osprey? Was he Jonah, swept into the jaws of a hungry predator? Or did he simply think, "Oh, boy, here we go again"?

When the Conservatives gather in Bournemouth in a few weeks, the snapshot of David meeting Nelson will be used extensively. We will, no doubt, be shown it during the seconds before Cameron bounds on stage to make his first leader's conference speech.

Each time the image flashes on screen there will come a few snide whispers of disapproval. One or two Conservative relicts and spotty hotheads who think South Africa has never been the same since that efficient chap Hendrik Verwoerd was in command. The majority of Tories are not bigots. That is what the David-Nelson handshake was partly intended to show.

It was also a case of "look, here's me being accepted into the top politicians' club". The two men, we are told, talked about HIV and Western aid, though it is possible the words "now remind me exactly who you are again" also came into the conversation.

There are Tibetan hermitages whose flagstoned floors have been worn thin by generations of barefoot pilgrims. The volcanic Black Stone at Mecca, the great doors of the Church of the Holy Sepulchre in Jerusalem, the Blarney Stone. All have been polished by the touch of centuries, coated by a patina of supplication. And so it must be with Nelson Mandela's right paw.