someone or something that is very good, pleasing, or useful难能可贵的人(或物);被人喜爱的人(或物);非常有用的人(或物)
He's a real gem - you'd be a fool to break up with him.他可真是个宝贝——你得是个傻瓜才会想要跟他分手。
The coffee house is a hidden gem (= something that not a lot of people know about).这个咖啡馆真是块隐藏的瑰宝。