Someone here advocated that a while back. I like the idea, partly as a way of carrying a magnet while keeping it away from credit cards(*) etc, but couldn't find a workable solution. When I asked about magnetic necklace clasps at a local jewellers, they told me they weren't reliable enough for anything of value.

(*) - I've seen a lot of discussion about how real the danger is, but I'm not yet convinced either way.