Rings were common throughout the Roman Empire, particularly in the later periods, when everyone was allowed to wear them, except slaves.
However, in ancient (Republican) times, the rings were usually worn by aristocrats and senators, in fact, wore a plain old iron ring, which was probably handed down from one paterfamilias to another over generations. If I recall, only the first eight senior most centurions were granted the privilege of wearing rings and I'm sure they were plain and not gem studded, as people would like to imagine.
Of course, aristocrats also had a signet ring, which was used to seal documents and evidence their signature.
well said skarr, i would like to add that a centurion ring was regarded as a priceless hierloom and was frequently past from genereation downwards, thi s is probably why many think that the it was either the firxt cohort (therefore 10 centurions) or 8 as you say and i agree.