If you really want to help Africa, Mr Blair, there is something that you could do...

Zsa Zsa Gabor once said: "I never hated any man enough to give him back his diamonds." As Tony Blair's Commission for Africa met for the first time yesterday, it is worth pondering her words.

The worst civil wars in Africa are fuelled by diamonds. African rebels buy their massacre-hungry weapons with the gems, and we in the West happily hand over the cash. Diamonds are not an African's best friend: the almost biblical slaughter in the Democratic Republic of Congo - where three million people have been murdered in the past five years - is funded by rocks glistening on the fingers of Western women. If the rest of the world stopped buying these diamonds we would choke off much of the killing. African militias really are getting blood out of a stone.

If the Prime Minister is serious about helping Africa, here is one hard, practical policy area in which he has the power to make a difference. The world's diamond industry operates out of London. Sixty per cent of all rough diamonds make their way through our capital. Tony Blair has to decide: does he love Africans enough to make us leave their havoc-wreaking diamonds alone?

The central myth of Blair's premiership has been the belief that you can improve the lot of the poorest without challenging the powerful; that we can all move forever upwards on the benign New Labour escalator. Sometimes it works, but more often it fails. To "sort out" Africa - as he says he wants to - Blair would need to challenge some powerful forces. He could face down the International Monetary Fund about the disastrous market-fundamentalist programmes it forces on African economies. He could face down our own heavily subsidised farmers, explaining that their cash payouts are starving African farmers by making it impossible for them to compete. He could stop licensing British arms companies to sell expensive military technology to countries with starving populations.

There was no mention of any of this at the launch of the commission yesterday. And there was silence when it came to diamonds. Here Blair has a stark choice: does he act in the interests of a powerful lobby serving billionaires, or the interests of millions of the most desperate people with no lobbyists and no voice?

His record on conflict diamonds is not entirely bad. The Government played a key role in pressing for the Kimberly Process, a system of voluntary self-regulation for the diamond industry. The process was finally formalised last year, when the diamond trade agreed to stringent rules.

But the very idea that this industry could be trusted to regulate itself was always - as so often with Blair's actions - too optimistic. The amoral core of the diamond industry can be seen in its recent past. Just over 150 years ago Cecil Rhodes founded the industry in its modern form by cleansing areas he thought might contain diamonds of the local "niggers", taking them back only as slaves to work in his mines.

The continuities from that period to this are striking. Ethnic cleansing of people unlucky enough to live on top of potential diamond mines continued until the 1980s. Sections of the diamond industry were severely condemned by the UN as recently as 2000. Is the slaughter of black Africans still - as it was for Rhodes - a negligible concern when there are gems to gather?

The evidence about the voluntary approach is now in. Global Witness, a Nobel peace prize-nominated organisation, has conducted an extensive survey of the sale of conflict diamonds in the US, where 50 per cent of all diamonds are sold. The implementation of the Kimberley principles was "abysmal", they found, and even now, "five years after the conflict diamond issue came to the forefront, a large proportion of the industry is still in denial that there was ever a problem. It has instead focused its energy on a public relations campaign to try to make the issue go away."

When the diamond companies lie, Africans die. But just in case you think this is only a matter for Africans and bleeding hearts, there's more. It has been demonstrated beyond doubt - by The Washington Post and others - that al-Qa'ida has been using diamonds to finance terror cells since at least 1993. The US General Accounting Office conceded in November 2003 that diamonds were being used by al-Qa'ida to "earn money, move money and store money". Yet the Bush administration has done almost nothing, allowing the billionaires of the industry to carry on "voluntarily" policing themselves.

In the absence of our leaders acting to stem a major funding stream for terrorists and murderers, we can all work individually to stigmatise conflict diamonds. Nobody would be so stupid as to walk down a British high street wearing a fox-fur, or turn up at a party wearing mink. They would - quite rightly - be barracked and harangued. We need to start doing the same with people who wear diamonds. Demand to know: where did you get your diamonds? Do you know their origin?

I fear, however, that it will be harder to get people to sympathise with black Africans than with cute fluffy foxes. But Blair is in a perfect position to change the trade in conflict diamonds more quickly than the slow cultural shift that we can effect. Here is one of his favourite themes, the idea that terrorism and global poverty are intertwined. Here's a way to fight terrorism that also benefits some of the worst-off humans alive. Here is a global industry over which he has more power than any other world leader. He has no excuses.

The programme is simple and radical: no more relying on the benevolence of the diamond industry. The only way to enforce the Kimberley principles is to give them legal force. Blair can institute a legally enforced ban on conflict diamonds being sold or treated in Britain, and encourage the spread of this ban, just as he championed the landmine ban now ratified by 140 nations. He can require that every diamond sold in Britain be strictly licensed to show it is not from a conflict zone. Throw anybody who breaks the rules into jail for aiding and abetting murder.

Blair has the power to begin right here, at the glittering, festering heart of the industry. One Bill through parliament and he can create a model that would soon spread throughout the democratic world. Of course the diamond industry would howl, but its voluntary framework is failing. If Americans were dying, would we even consider making regulations to save their lives contingent upon voluntary corporate benevolence? Any petulant complaints from the diamond industry will only make the demand for a global ban even greater.

Blair can go further and echo the call of several development charities for anybody caught trading in conflict diamonds to be tried before the International Criminal Court as accomplices in ethnic cleansing and mass murder. The effect upon some of the worst conflict zones in Africa would begin to be felt within a few years. Now, that would be a legacy; that would be an Africa commission worth leading.

j.hari@independent.co.uk