Of all the gin joints in all the war zones in all the world...

Blood Diamond is a movie that tries to be many things at once and succeeds at none of them. It is directed by Edward Zwick, whose previous films Glory, Legends of the Fall, and The Last Samurai are about individuals caught in critical moments in history, often unwillingly, who then surprise themselves by doing something great. To my mind Zwick has never made a great movie, but he does churn out thoughtful entertainments. This time Zwick takes on the civil war in Sierra Leone in the 1990s, and the illegal trade in Sierra Leonean diamonds that funded that war. The diamonds were smuggled across the border to Liberia, where they were labeled Liberian diamonds and sent on to Europe. From there they would make their way onto the ring finger of some American bride (or into a vault, to limit the supply of diamonds and keep the prices up).

The individual caught in this historical moment is Danny Archer, a South African mercenary and diamond smuggler played by Leonardo DiCaprio.

DiCaprio has always been an interesting performer, but with Martin Scorsese’s The Departed he finally made the transition from pretty child to man. In Blood Diamond we can believe he is a former professional killer and opportunist. He’s very good in this movie, and more importantly he doesn’t go whoring for an Oscar. ("Look at me, Oscar voters! See the vein pulsing in my forehead, gimme a trophy!")

Archer meets Solomon Vandy (Djimon Honsou, who is always cast as the noble suffering man), a fisherman who had been taken from his family by rebels and forced to work in the diamond mines. There he found a humongous diamond, which he buried in secret. Vandy needs to find his family, and Archer wants the rock. To retrieve it they will need the help of an American journalist named Maddie Bowen (Jennifer Connelly), who is doing a story on the illegal diamond trade.

So we have the classic Casablanca set-up in which a cynical man who only cares about himself is spurred on by love to heroism and self-sacrifice.

Except that the Danny-Maddie romance never really ignites; we get a few flirtation scenes, and some soul-baring over palm wine, but we’re not convinced. And if two beautiful people in the middle of a war zone can’t generate any heat, what is Hollywood for?

Zwick throws in many horrific gun battles, people shot at close range, explosions – we figured he’d ditched Casablanca for Saving Private Ryan.

Then there’s a long sequence in a camp for children who had been kidnapped by rebels and forced to become soldiers. It is one of the gut-wrenching horrors of our time, children being kidnapped and forced to kill people, but in this movie they’re just atmosphere. This sequence had the feel of a UNICEF ad: "Give to the suffering children of Africa, you prosperous well-fed non-sufferers." (Angelina Jolie quip erased.)

Blood Diamond is shapeless and interminable; we don’t know where the movie is going, which is all right as long as we can trust the director to lead us there. Instead, he tries all sorts of unnecessary detours – the movie is at least 40 minutes longer than it should be. At one point I was seized by nostalgia for the movie Congo, which was also about a search for a huge diamond. In fact I began to hope that a trained gorilla would suddenly appear and say, "Amy, good gorilla!" That would have relieved some of the tedium. Why must movies about important subjects be so tedious? Is tedium an indication of gravity?

Despite its stellar cast and vital subject matter, Blood Diamond does not really engage its audience. My mind drifted off and I started thinking about a movie called Sands of the Kalahari that I saw on TV as a kid. It was about people who smuggled diamonds out of a mine by swallowing them. Somehow they end up in the desert and are set upon by evil baboons.

When my mind returned to the movie it had turned into The Lord Of The Rings – Archer and Vandy are climbing sharp rocks, and Vandy stretches his hand out to the injured Archer. DiCaprio has a big scene in which he finally looks at the diamond that may cost him his life. He holds the rock to the light – then the director cuts away to the approaching soldiers, and the emotion is lost in a hail of bullets.

The screenwriters have no idea how to end the movie, so the scene shifts to London. We are looking at a diamond necklace in a jeweler’s window.

"The movie has morphed into The Thomas Crown Affair," sighed Ricky.

But it’s not over yet! There’s a sting operation to trap a prominent diamond merchant. "Now it’s James Bond," said Chus, who’d spent the last hour checking his watch. "Shirley Bassey should come out and sing Diamonds Are Forevah." Now there’s an ending.