Leonardo di Caprio is Danny Archer, a scummy guy and former mercenary who's currently in the diamond-smuggling business. He doesn't care where the diamonds come from. He just wants to make money. When he hears that a fisherman named Solomon has found a 100-carat pink diamond, he wants in.
The problem is that Sierra Leone is in the midst of a horrible civil war during the time the movie is set. The diamonds smuggled out are being used to finance the war, which is presented in gut- and heart-wrenching realism. Violent deaths and mutilations are as common as air. If you've bought a diamond lately, you're likely to feel a little bit guilty, especially if you didn't know about "conflict" (or "blood") diamonds.
The message doesn't overwhelm the story, though. Danny wants the diamond, Solomon wants his family back, and Maddy, the journalist played by Jennifer Connelly in a thankless role, wants a story. They all get what they want in one way or another.
My main quibble with this movie is that it went on for fifteen minutes after it ended. I suppose the final quarter hour was tacked on to make you feel a little better. Didn't work for me. It was redundant, and it felt it. I'd still recommend the movie, though.