I would say that there are two main ideas in De Maupassant's story and both are entwined and reinfoce one another. The questions to be considered are: what is beauty? and what is wealth?
For example, one begins to question beauty. Is it really all on the outside? Madame Loisel is beautiful, but she is not content. She has the appearance of beauty but not the reality of beauty.
One also begins to question greed. Living modestly with her husband before the ball, Madame Loisel believes she is suffering a terrible injustice by having few luxuries. In fact, she does not experience the reality of poverty until she and her husband go into debt to pay off the necklace.
"The Necklace" is a social re-evaluation of values, both material and internal.
In 'The Necklace' Madame Mathilde Loisel is unhappy with her position in life, she dreams of a life of leisure and of being well-off - but the reality is that she has a modest life style. She has stopped visiting an old friend Madame Forestier who has become wealthy because she feels ashamed of her social standing.
Mathilde sees the ball she and her husband are invited to as a way to the life she dreams of. Her husband does not purchase the gun he has been saving for to buy her new gown for the ball, and her husband suggests she borrows jewels from Madame Forestier when Mathilde laments that she has no jewels.
Mathilde is a success at the party, but when she and her husband return home she realizes that she has lost the necklace that Madame Forestier has loaned her. Despite their searching, they are unable to find it. Mathilde and her husband go deeply into debt to purchase a new necklace for Madame Forestier and spend years working the debt off at a great cost to themselves, only to find out once the debt has been paid off that the original necklace had been a fake worth only 500 francs.
The main idea of the story is that Mathilde's greed, dishonesty, and desire for a better social position in life ultimately led her to a life much worse than she originally had. If she had never felt it necessary to borrow the necklace so she could look wealthy, had been honest with Madame Forestier that the necklace had been lost, Mathilde would have never ended up where she did.