I can see it now: two rings, one rubber to represent user requirements, and the other silicon to represent the systems we have to implement those requirements in.

Glass ring encased in rubber, with a label claiming it is exactly as flexible and stretchable as a 100% rubber ring would be.

I am getting somewhat jealous now – people should make wearable cyclic particle accelerators or something like that for those hacking nature rather than mere computers :-)

Diamond made out of isotopically pure carbon-14. Ring laser gyroscope. Negative-refractive-index metamaterial. A single giant LED. Nanolithography to produce structural coloration.

I've wanted a cool physics ring for quite a while.

It also should be able to reverse the polarity of the neutron flow. Clearly, that is a key requirement.