The Question and Answer section for The Necklace is a great resource to ask questions, find answers, and discuss the novel.
A naturalist believes that there are no spiritual or supernatural laws in the world, that there are only "natural" forces..... think nature.
I think that Mathilde Loisel was so self-absorbed and lost in her fantasy that she did not consider that the necklace could be anything but real. Her husband was just trying to appease his wife so he did not give the necklace a second thought.
It turns out that the necklace they were paying back for ten years was actually a fake.