This is a necklace used to store a ring securely when not wearing it on your finger. Thread a chain through both of the loops at the top and fasten around your neck as normal. To put the ring on, just push the ring over one end of this loop, then move it around the entire loop (see pictures). When done, the ring will rest on top of the loop, in a way that is very resistant to being knocked free.
I designed this in CREO parametric. I printed several prototypes on my reprap 3d printer to get the dimensions correct, but the real version in premium silver looks amazing. The ring shown here is platinum, which matches the silver very well (I can only tell the difference in metal under a good light and a magnifying glass). I would recommend the premium silver, as the sheen is lovely, although one of the gold versions would be a better choice for a gold ring.
I made this for my fiancées engagement ring, it is a size K (UK size), and the band is approximately 2.5mm wide. However, the size is not too critical, and any ring that is not very wide will fit nicely.