In Arthur Miller's Death of a Salesman, Willy Loman has been a salesman for over thirty-five years. Now in his sixties, the job is very hard for him because he is still on the road for long hours. He is also very concerned about his children and their success in life, especially Biff from whom he has always expected great things. However, Biff has not been successful; Willy lives so much in the past, that he overlooks his son's failure to graduate his senior year and the loss of his scholarship.
Willy's other difficulty is being so caught up in the past (in the way things were and the way his life could have been) that he has imaginary conversations with his brother Ben. Ben was a great success in life. One way he made a fortune was going to Africa and coming home with diamonds.
...instead I ended up in Africa.
The Gold Coast!
Principally diamond mines.
...Boys! Listen to this. This is your Uncle Ben, a great man! Tell my boys, Ben!
Why boys, when I was seventeen I walked into the jungle, and when I was twenty-one I walked out. And by God I was rich.
Several minutes later, Ben repeats himself:
William, when I walked into the jungle, I was seventeen. When I was walked out I was twenty-one. And, by God, I was rich.
The repetition indicates that this is an important point that Ben is sharing with Willy—and the audience. Literally it shows that Ben was a success within just a couple of years, though it was a different time, and much easier to do. Part of Willy's problem is that he is stuck in that time. Willy believes there must be a way for his boys to do the same, if they could be more like Ben. And Willy wishes he was more like Ben. He admits to Charley that his brother tried to get him to go to Alaska and make his fortune in timber, but Willy never went.
Willy's father went to Alaska and never came back. Ben was older so he knew his dad, and may have caught his adventuresome spirit, but Willy was almost four when his father left and hardly remembers him. Willy never had an adventuresome spirit, choosing to play it safe and find a regular job, but this has been a disappointment to Willy, and he has many regrets.
Even when Ben gave him a pocket watch with a diamond in it, Linda has to remind Willy that he ended up selling it at a pawn shop. This demonstrates how difficult Willy's life has been, and how different it is from his brother's life. Even as a gift, Willy was unable to hold onto that diamond.
With the near-immediate success that Ben achieved mining diamonds in Africa, the diamonds become symbolic of success for Ben, and missed opportunity for Willy. Sadly and ironically, Willy finally gets his sights on a diamond at the end of the play what will make money for Linda, and make his boss appreciate him; both perceptions are flawed.
Oh, Ben that's the whole beauty of it! I see it like a diamond, shining in the dark, hard and rough, that I can pick up and touch in my hand...the funeral...will be massive! They'll come from Maine, Massachusetts...All the old timers...[my boss] will be thunderstruck...because he never realized—I am known!
The "diamond" is a suicide plan: Linda will get his insurance. And the funeral, he believes, will show his boss how successful Willy really was...except that with suicide Linda probably won't get any life insurance; and no one will come to the funeral—the old days are gone. Even in death, Willy is unsuccessful.