What is situational irony in "The Necklace"?

Situational irony takes place when actions have an opposite effect from what was originally intended. In "The Necklace," Mathilde, despite her relatively lowly station in life, thinks herself so much better than her surroundings. She does not just aspire to better things; she believes herself entitled to them. To her, it is just a matter of time before she attains the place in high society which is her due.
But Mathilde's shallowness and snobbery lead to her demise. She thinks that wearing an expensive diamond necklace will make her the belle of the Education Ministry ball. But she subsequently loses it and forces herself and her husband into poverty trying to pay for a replacement. When she finds out that the necklace was a fake, the situational irony has come full circle. Mathilde desperately wanted to wear the necklace to improve her social standing, but now she is worse off than she was before and occupies a lower social class.