If only you know which are the valuable pieces, you really need to do something to make sure they are not sold for a pittance. Even a charity shop may just mistake then for costume jewellery. You could write out some kind of document, to be stored with your will (or a copy) explaining, perhaps with photos, which are the valuable pieces, to guide your executors.
Then you can instruct your executors as to how you'd like them to dispose of the items. Depending on who you have chosen, you could give them absolute discretion as to how to share these items among family and friends. (I have recently had to do just this in the capacity of executor). Friends might appreciate a single item to remember you by. Or you could have the best items valued and tell your executor to sell them and return the proceeds to the estate.
Cheaper costume jewellery is best given to a charity shop. If you put it in an auction it will raise next to nothing.
It might be more satisfying for you, and easier for your executors, if you can pass on some of the jewellery in your lifetime, perhaps giving the valuable items to mark a special date or anniversary. If there is stuff of little value that you don't wear, hand it over now to a charity shop of your choice.
My maiden aunt had a triple row of pearls that she kept in the bank. I always thought this was ridiculous; if she liked them, she should have been wearing and enjoying them. In the end, I inherited them and sold them at auction, as I had no used for them. My advice is either to enjoy your possessions or to pass them on to others if you no longer use or need them.