Diamonds may be a girl's best friend but don't ask any of the women and children who had their hands chopped off in the deadly world of the international gem trade.

This was the horrific tactic used by rebels in Sierra Leone who traded so-called "conflict" diamonds - farmed illegally from rivers - for arms in a brutal civil war fought by brainwashed child soldiers.

Ed Zwick's Blood Diamond exploits the action thriller genre to lift the lid of this odious trade that fed into 15% of the global diamond market in the late 1990s.

Leonardo DiCaprio gives a mesmerising performance as cynical one-time South African mercenary Danny Archer who acts as a go-between for a diamond company in London. Captured by government forces in Freetown he hears of tribesman Solomon Vandy (Djimon Hounsou) who escaped from the rebels and is hiding a huge pink stone he found while forced to work for them.

The two men get together to retrieve it, Archer to buy his way out of Africa, Solomon because he wants to bargain the release of his family from a refugee camp and rescue his abducted son. An investigative journalist (Jennifer Connelly), links up with them, completing the ingredients for a spectacular blockbuster.

Along the way Archer develops something of a conscience ("Sometimes I wonder if God will ever forgive us for what we've done to each other. . . Then I look around and I realize, God left this place a long time ago"). It turns out he's the way he is because he saw his parents butchered as a child. "Mum was raped and shot and dad was decapitated and hung from a hook in the barn. I was nine. . . boo-hoo, right?"

In some ways Blood Diamond is the reverse of The Last King Of Scotland, a perverted buddy-buddy relationship in which the white man is evil and the black man noble. But in Africa nothing is black or white, and Oscar-nominated DiCaprio brilliantly captures this moral ambivalence as a killer who seeks redemption.

Ed Zwick doesn't fully resolve the contradictions between the massentertainment format and the challenging political and ethical themes, but by raising them so unflinchingly he lifts Blood Diamond out of the Hollywood norm.