M. Lantin asks about the necklace's value due to his disbelief in its true worth.
M. Lantin asks the first jeweler if he is sure about the value of his wife's necklace because:
When M. Lantin finds out the true value of the necklace, it comes as a shock to him as he had believed it to be of little worth. This reaction showcases his disbelief and astonishment at the actual value of the necklace.
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