I used to write wills for a living and I know that the inclusion of any 'provisional' clauses in a will can create all sorts of problems.
If I was asked to draft your will, I'd be reaching for the reference books in a quest to find the best possible wording. My first thoughts though are that you shouldn't actually bequeath the jewellery to your husband (unless you simply tell him that you'd like the jewellery to go your niece if he doesn't want to hold onto it, or to sell it). It would be better to simply give him the right to claim the jewellery, roughly along the lines of the following:
"I grant to my husband, John Albert Smith, the right to claim title to any or all of such items of jewellery as I may own at the time of my death. Should my husband, John Albert Smith, decline to claim title to any or all of such items, title in any unclaimed items shall pass to my niece, Brenda Jayne Parkin, of 42 Acacia Avenue, Littlehampton".
NB: That's very much a first draft but it's a (very rough) guide to the type of wording that a solicitor might consider if it meets with your needs.
Chris