You can purchase additional insurance for your engagement ring through your renters
or home insurance
. It would be through a rider or additional endorsement if the value of your ring is greater than your property limits (which it very well could be). You will usually need to provide an appraisal to your insurance company so they know how much to insure it for. In terms of who insures it, it can depend. If you bought it independently, I recommend that you insure it yourself as you are the original owner.