Take them to an auction house and ask what sort of reserve price you should put on them for their next sale. If you don't like the price suggested, you don't have to put them in the sale. If you ask for a written valuation, however, the auctioneer will do it for you, but probably for a percentage of the value. If you do go for an auction sale, wait until they are having a specialist jewellery sale, as there will be more buyers then.
A jeweller will also give you a valuation - also probably for a percentage. Could be up to ten percent of the value
One note of warning - it is difficult to sell second-hand jewellery except by auction, as jewellers fear you may have stolen the stuff.