What lesson does "The Necklace" teach?

There are a number of lessons in Guy de Maupassant‎’s short story “The Necklace.” Two that are prominent are “all that glitters is not gold” and the best policy is to tell the truth. Mathilde grew up in a simple background but longed to live an upper class life. She married a working class gentleman who did his best to please her.

When her husband procures an invitation to a dance she is distraught by her lack of appropriate attire and jewelry. Her husband gives her his savings to purchase a dress and suggests that she wear flowers or borrow some jewelry from her friend. She borrows a sparkling necklace from her wealthy friend, which she assumes is diamonds and gold. Unfortunately, in a haste to leave the dance because she is embarrassed by her shabby wrap, she loses the necklace. Instead of telling her friend the truth about losing the necklace, she and her husband purchase a replacement which takes them ten years to pay off.

Mathilde and her husband lose their home and youth paying off what they thought was a diamond necklace. When Mathilde has a chance meeting with her friend after finally paying off the necklace, the friend reveals that is was a fabulous fake and not the real thing. If Mathilde had told her friend the truth when the necklace was lost after the dance, there may have been hurt feelings but it would have saved Mathilde and her husband ten years of turmoil.