there's a difference between having it specified on your house contents and having all risks cover.
having it specified on house contents simply tells insurer that there is one item of contents valued at €XX. The cover is only the perils specified in the policy applying to contents.
If you have lost a stone, then the ring needs to be under your 'all risks' section of the policy. get an estimate for replacement stone and submit to insurer if you have all risks cover. with some insurers, once a valuation was produced first day, then there is no need to get another valuation of the item. With others, you will need a current valuation.