someone or something that is very good, pleasing, or useful難能可貴的人(或物);被人喜愛的人(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.他可真是個寶貝——你得是個傻瓜才會想要跟他分手。
The coffee house is a hidden gem (= something that not a lot of people know about).這個咖啡館真是塊隐藏的瑰寶。