What makes the story's closing sentences ironic?

The closing sentences are filled with irony because Madame Loisel had spent ten years to replace a necklace she believed to be real.... when the original was a fake. Never would she have dreamed that her wealthy friend would lower herself to wear something that wasn't genuine... she thought wrong.

"Oh, my poor Mathilde! Mine was an imitation! It was worth five hundred francs at most! ..."