Not many of the tourists who go to see the Crown Jewels in the Tower realise that two of the most magnificent stones are from Afghanistan, the Black Prince’s Ruby and the Timur Ruby. Henry V wore the two-inch-long Black Prince’s Ruby on his helmet at Agincourt. It is now the centrepiece of the Imperial State Crown. The Timur Ruby is even larger and carved with the name of Tamerlane and five of its other keepers. Both almost certainly came from a mine in the Pamirs first described by Marco Polo, in one of the most inaccessible parts of Afghanistan. Gary Bowersox employed old maps and his considerable courage to rediscover it, trekking with horses up the rocky Oxus Valley and across passes christened, by the Danish expedition that first mapped them, Devil’s Pass One, Two and Three.

Too often travel writers confect exquisite experiences out of very little, but Bowersox has, if anything, too much experience to fit easily into a book. He has been travelling in Afghanistan, where he is a well-known figure, for thirty-five years. He served in the US Army during the Vietnam War and is large, tough, generous and unassuming. I first met him in 2001. Afghanistan is a big place and I asked how I would find him. ‘Just ask any of Commander Massoud’s men for Mr Gary,’ he said. And when I pitched up at Massoud’s HQ, I was told he was in the room next door.

Known as the Lion of the Panjshir, Ahmed Shah Massoud was the greatest of all the mujahedeen commanders, and with his appearance the book really gets into its stride. Up to this point, it has been an account of Mr Gary’s career as a gem wholesaler, but then he receives a telegram: ‘Massoud interested in our gem programme for Afghanistan.’

Bowersox’s description of Massoud does justice to the man. ‘His eyes were alert but not at all shifty. He had an air of composure and not a hint of aggression. His firm strength gave strength to me. He seemed to draw power from somewhere else: from the earth beneath and the mountains beyond. I felt an energy I could not describe but I knew that there stood before me an extraordinary man.’

As a pendant to the figure of Massoud is Osama bin Laden. Bowersox is one of the few Westerners ever to have encountered him, in Green’s Hotel in Peshawar, the legendary North West Frontier hang-out of spies, soldiers, arms dealers, and journalists. Again, Bowersox’s writing rises to the occasion. ‘His thin, six-foot-four frame made him look otherworldly, like a saint painted by El Greco. I felt no sense of communicating with another human being. He was off somewhere in his head. He had less presence than a specter.’

The narrative is structured round a series of trips to the major gem-bearing areas of Afghanistan during the Afghan civil war of the 1990s. Each time, Gary had to smuggle himself across the border, and he had a series of bizarre adventures that any travel writer would kill for. On the first trip he was badly injured by a fall from a horse in the middle of the night, and on the next he was hidden in a metal box with a person suffering from giardia. In 1999 he and his team of prospectors were caught in a Taliban offensive that displaced one hundred thousand refugees. Their jeep pushed its way through the crowds and they made it just in time to a helicopter that Massoud had sent for them, which turned out to have been captured from the Taliban a few hours before. In another cliff-hanging denouement he was surveying a ruby mine controlled by the Taliban and had to flee, finally crossing the Khyber Pass disguised as a woman in a burqa. ‘My wife is very tall,’ explained his guide to the burqa-seller. ‘I want a divorce as soon as we get to Pakistan,’ said Gary.

But it is not all John Buchan. Gary is sensitive to the atmosphere of this extraordinary country as he walks silently with guards through the mountains: ‘the stars were penetratingly clear. It was a peaceful time that I shall never forget.’

The other theme of the book is the idiocy of US policy in Afghanistan over the years. There must be very few, if any, people other than the author who have observed so closely the new Great Game that the CIA has been playing in Afghanistan since the start of the Russian jihad. They subcontracted arming the mujahedeen to the Pakistani secret service, the ISI, who pursued their own divide-and-rule agenda by arming seven competing factions. The most grotesquely wicked recipient of their largesse was Gulbuddin Hekmatyar, who has repaid the Americans by defecting to Al-Qaeda and emigrating to Pakistan, from where he launches butcher-and-bolt raids into Afghanistan. And the CIA would have nothing to do with Massoud, interdicting even humanitarian aid to his area. It is not only with hindsight that this policy appears stupid. I remember in 1999 asking Massoud’s brother why their troops did not snatch Bin Laden and make a present of his head to the Americans. His reply was depressing and astonishingly prescient. ‘Because the Americans would say “Thank you very much” and walk away. No, what we need to do is get them to see that Bin Laden is their problem and not just ours.’

In August 2001 I was in Afghanistan, delivering medicines to a hospital near Massoud’s HQ where the American interdiction meant they were performing operations without anaesthetics. On 1 September at a bleak mountain village I waved Gary off, who was mounted on a horse, at the start of his three-day journey through the most spectacular mountain scenery in the world and into Pakistan. On 6 September 2001 he met with State Department officials in Washington, proposing a meeting between Massoud and Colin Powell. ‘There is no interest in such a meeting,’ said the desk officer curtly. It would, in any case, have been too late. Three days later Massoud was dead, killed by two of Bin Laden’s suicide bombers in an attack that was almost certainly the cue for the atrocities in New York on 11 September. Then the Americans found themselves desperately looking for a credible Muslim ally to overthrow the Taliban. It would be reassuring to report that they had learned their lesson, but a reader of this book is not left optimistic.