Are there any Jewish people here? I have some questions...

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If anyone who is Jewish is willing to answer my questions, I have a few. They're mostly cultural (as far as I know), and they're not derogetory in any way-I'm just curious about some the practices. For example, what does the practice of placing stones on the grave sight or headstone of the deceased symbolize? And regarding the preparation of the body (for burial), what is "allowed" and why? Are autopsies, embalming or cremation allowed?