Blood Diamond (15) <!-- none onestar twostar threestar fourstar fivestar -->

Diamonds have been outed. Once known as a girl's best friend, they will (for a while at least) find the going tough, impugned as the rocks on which whole countries have been broken into pieces. And, by an irony of hard, gemlike dimensions, it's a movie straight out of Hollywood, the capital of bling, that will do the most to highlight their flawed reputation.

Blood Diamond isn't a great movie by any stretch - it's overlong, and it trades in too many generic clichés - but it is purposeful, courageous even, in addressing a complicated issue of exploitation and trying to make it comprehensible, all within little more than the time it takes to munch your way through a vat of popcorn.

Set during a bloody civil war in Sierra Leone in 1999, it concerns the wildly contrasting fortunes and outlooks of two Africans, one black, one white. Solomon Vandy (Djimon Hounsou) is a fisherman who sees his village razed by a guerrilla army, the RUF, and his son dragooned into their ranks. Having narrowly escaped a machete severing his arm - the ultimate voter disincentive - Solomon is set to work in the diamond fields by the guerrillas, and by chance dredges up an absolute beauty, which he buries nearby.

Eventually, word of this diamond, not quite as big as the Ritz but valuable withal, reaches one Danny Archer (Leonardo DiCaprio), a white mercenary from Zimbabwe who knows all the tricks of this hugely dishonest trade. Danny springs Solomon from the jail they both happen to have landed in, and strikes a deal: if he takes him to the diamond, Danny will help to recover Solomon's son, who by now is being trained as a child soldier in the RUF. (These scenes are, against stiff competition, the most horrifying in the film, for they describe precisely how rebel militias bully and cajole pubescent boys into the ways of remorseless killing).

Leavening the macho mood of adventure comes a war reporter, Maddy Bowen (Jennifer Connelly), who wants to get the inside story on the diamond-trafficking between Africa and Europe. She knows that if anyone can provide the scoop it's Danny, but he knows that if the Western media ever rumble his paymasters he'll never work, or indeed live, in Africa again.

Maddy offers sobering perspective on the tragedy of this war-torn country - "You might catch a minute of this on CNN, somewhere between sport and weather" - but something about her commitment and decency touches Danny. Then again, it might just be something about her looking like Jennifer Connelly.

Too often the pale, martyred companion to control-freak supermen (A Beautiful Mind, Pollock, The Hulk) Connelly actually seems to enjoy her independence here, and flirts with a spirit that lightly mocks men and her own power over them. Danny responds to her and very reluctantly comes round to the idea that he might have a conscience after all. But he won't let her take all the moral high ground: as he reminds her, glossy ads for diamonds proliferate in the very magazines she's writing for.

DiCaprio follows up sterling work in The Departed with another career high. Having shed his puppyish prettiness, he looks better with a few years on him, and there's a foxiness around the eyes that make you think he's exactly the kind who would thrive in this moral quagmire. He's also put on a pretty convincing Rhodesian accent, biting off monosyllables with gruff relish.

Hounsou has the least satisfying role, because he has the most nobility to carry; every 10 years or so, one actor has to shoulder the burden of the hard-pressed but high-souled black man, and now that Denzel Washington is too old most of the responsibility has passed on to Hounsou. I hope some director casts him as a villain soon - he deserves a break.

The uneasy truce binding Danny and Solomon together on their way across the perilous, guerrilla-infested terrain may remind you of Sidney Poitier and Tony Curtis in The Defiant Ones, and hints at director Edward Zwick's fondness for tales of racial brotherhood (he made the awful Tom Cruise-goes-native movie The Last Samurai). But the script (by Charles Leavitt) also provides them with at least one ferocious barney, during which Danny credibly reverts to type and calls Solomon a "kaffir". So much for Truth and Reconciliation.

There are moments of wishful thinking and silliness that we could moan about, most of them in the last half-hour when the wheels of socioeconomic justice supposedly grind into action. But there are worse things to be than a liberal, and the film-makers, while determined to prick the West's conscience about Africa ("Let's hope they don't discover oil here," says an old refugee) never forget that they're making an entertainment, too.

Eduardo Serra's photography is wonderfully crisp, and the jittery hand-held shooting of the street massacres packs a horrible immediacy. The amount of wanton, indefensible barbarity is kept vividly before us, a state of being that characters allude to as TIA - "This Is Africa". Blood Diamond at least acknowledges the chaos into which it has strayed, and makes sure that we take seriously its argument for responsibility.

Some will dismiss it as "typical Hollywood", yet that isn't always a bad thing. In so many movies, destruction and loss are dwelt upon because they are irreducible truths. But survival is a truth, too, and this film pays it often stirring witness.