What lessons were learned about the diamond necklace in Guy de Maupassant's "The Necklace"?

One of the lessons learned in Guy de Maupassant's short
story, "The Necklace," is that of the dangers of extreme personal vanity. Madame Loisel
was single-mindedly absorbed with the idea of the ball to which she and her husband were
invited. It was to be the greatest night of her life, and she was willing to allow her
husband to spend the family's savings on a gown that she considered necessary for the
function. Yet, that was not enough. Expensive accessories were also needed, so she took
the step of borrowing what she thought was an appropriate necklace for the finishing
touch to her ensemble. She could not focus on anything beyond the ball--it was to be her
greatest moment. Ironically, the necklace, which proved to be paste, fooled everyone,
including Mathilde. The loss of the necklace certainly brought both of the Loisels back
down to earth, yet her own vanity would not allow her to admit to the loss of the
necklace to its owner. A simple admission of truth to her friend would have solved the
problem and saved them the years of toil that followed.