a gem

An especially wonderful, excellent, or exemplary person or thing. I knew my husband was a gem from our very first date. He was so kind and generous. You're a real gem, Sarah, you know that? Not many people would offer to stay late each night to help me with all this extra work. Oh, this film's a gem. Audiences will treasure it for years to come.

a gem of (a person or thing)

An especially wonderful, excellent, or exemplary person or thing. Ashwin is really a gem of a man. He has done so much for everyone in our community. It proved to be a gem of an idea that brought the company back to prosperity. You're a gem of a person, Sarah, you know that?

hidden gem

That which is of exceptional or underappreciated quality but is not especially popular or widely known. It may not have been a huge success, but this film is a real hidden gem from the 1980s. The reviewer calls the restaurant one of the hidden gems of Manhattan. Did you see that antique vase? Molly always finds the best hidden gems at the thrift shop!