Unfortunately, the best answer one can give you without more information is "it depends." The specific coverage will be found in the language of the policy itself, and different policies provide different coverages.
I've written a brief legal guide regarding coverae of jewelry under homeowner's insurance policies, which can be found here on AVVO, or you can link to my blog here http://law-wi.blogspot.com/2008/08/jewelry-loss-claims-under-homeowners.html
In the article I provide a primer on the coverages available under different policies and how those coverages may apply to different types of jewelry losses.
Here is the link to the AVVO legal guide on this topic: