Sterling and Argentium Sterling are both nickel free. Some fine jewelry store sterling is rhodium plated - there is a faint chance that it could have nickel under the rhodium. nickel free
Niobium and Titanium are nickel free, non-reactive and very often used for people with concerns about metal allergies.The RIng Lord has titanium jump rings. They may also have titanium clasps... yes, they do at Titanium lobster clasp.
Gunmetal platings are usually a black nickel finish so you should avoid gunmetal platings. White gold, also, is usually made with quite a bit of nickel in the alloy.
Surgical Stainless Steel alloy contains nickel but very few nickel ions travel to the surface to irritate the wearer. Euro standard Stainless steel has an even lower level of free nickel ions to reach the wearer. It is widely available in UK and Europe but not so common in the US. We DO carry both kinds of Surgical Stainless Steel at Rings & Things. See R&T Findings for several pages of findings. (A few nickel plated items also show- just skip over to see the ones marked "nickel free.") Our magnetic chokers are also nickel free as well as all our cloth chokers.
It's very easy to make a button or bead and hook clasp that uses no metal at all.