There are two ways you can get a piece of jewelry insured.
The first way is to call your homeowners or renters insurance and ask them about the process of adding a jewelry rider to your existing policy. They will then go over that process with you, the premium costs, coverages, appraisal requirements, etc.
The second way is to get a stand alone jewelry policy. Some large providers of these stand alone policies are Jewelers Mutual and BriteCo, though there are many more options out there with a quick Google search. You can give them a call and they will go over the same items with you as above, the process, premium costs, coverages, appraisal requirements, etc.
I encourage you to do some research into the different companies and policies out there. I am a jewelry expert, not an insurance expert, so this blog is only intended to give you direction to do your own research to find the right fit for you.