is a diomand ring a symbol of a whole life?

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when the new couple get married,the man buy a diomand ring for the woman as a loving gift.in a famous ad,there is a saying that the diomand can last forever,one can be passed down forever.the diomand is the hardest thing in the world,people consider it to the true love for the other.but why do some couples get divorced though they buy each other diomand ring when they were married?we can get a conclusion that understanding and support is the guarantee of a whole life but not a diomand ring .