。
She has an unbelievably valuable ring.
Literal
She [topic-は] cannot-be-believed [to the extent-くらい] value-existing ring [object-を] is-holding.
信じられないくらい ('to an unbelievable degree') uses the potential negative of 信じる + くらい to create a degree expression. The whole phrase modifies 価値のある ('having value'), which in turn modifies 指輪 ('ring'). Multiple nested modifiers — a structure that showcases Japanese's ability to stack clauses before a noun.