She was gazing intently at the girl's beautiful ring.
Literal
She [topic-は] girl [possessive-の] beautiful ring [object-を] fixedly was-gazing-at.
じっと is an adverb meaning 'fixedly, intently, without moving' — it describes sustained, unbroken attention. Combined with 見つめる ('to gaze at'), it emphasizes how intently she was looking. The ~ていた sets a narrative scene of prolonged observation.