。
She's wearing an expensive necklace.
Literal
She [topic-は] expensive necklace [object-を] is-doing.
高価な ('expensive, costly') is the more written-leaning version of 高い ('expensive') — it focuses on monetary value and is common in formal contexts. 首飾り ('necklace') is a native compound: 首 ('neck') + 飾り ('decoration, ornament'). The loanword ネックレス is also widely used in modern speech, especially for Western-style necklaces; 首飾り can lean slightly more traditional or generic. As with belts and other accessories, ~をしている is the natural form for wearing.