。
She's wearing an expensive ring.
Literal
She [topic-は] expensive ring [object-を] is-doing.
指輪 ('ring') is built straightforwardly from 指 ('finger') + 輪 ('circle, ring'). Like 首飾り and ベルト, rings take する as the verb of wearing — Japanese marshals the all-purpose する for accessories that don't fit the body-region-specific dressing verbs (着る, 履く, かぶる). Engagement rings (婚約指輪) and wedding rings (結婚指輪) have their own established compounds, both common in modern Japanese discourse.