What's the Name of That Book??? discussion

This topic is about
The Moonstone
SOLVED: Adult Fiction
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SOLVED. Thriller/Romance/Suspense Book about stolen family heirloom during a family event and the events that surrounds each person during the family event. [s]
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***SPOILER ALERT***
The novel was set around a family (including a maid) who was in a gathering of sorts and during this event a lot of the family members. This was in a big mansion of sorts as well; during this event, there was a man and a woman who were attracted to each other (the man had a sleep-walking problem). There was a talk of an object (I believe it was either a necklace or a ring worth fortunes) that belong to the woman (if I am not mistaken).
In the night everyone went to sleep and in the morning the object had been stolen and the woman (crying) decided to leave the mansion immediately without speaking to the man. After this happened, the novel went in depth into each character at the event (with several twists and turns and suspects) but eventually, the culprit was revealed.
In the end, the man was the culprit however he did not know he stole the object due to his sleep-walking issue. He went into the woman's room and stole the object without realizing it thus the reason the woman cried and went away from the mansion.
Sorry -- that's all I can remember :| -- I hope someone out there can help me remember/find the book.
PS: If my memory serves me right, the book came with a green or blue hardcover