Titanium split rings are lightweight, rust proof, and non magnetic. They come in four sizes:

14mm O.D., 12mm I.D., thickness 1mm, weight .3g

19mm O.D., 15mm I.D., thickness 2mm, weight .8g

25mm O.D., 21mm I.D., thickness 2mm, weight 2g

32mm O.D., 26mm I.D., thickness 3mm, weight 4g

*NOTE: Titanium split rings do not have the same memory of steel split rings and may deform if stretched too far. The smaller the size, the less it can be opened, so keep that in mind when using these. I personally use some of these on my keys and have been able to put wide keys on the 32mm with no problem, you just have to be careful when opening them.