GEM

gemstone [noun] [C] - a jewel, esp. when cut into a particular regular shapeShe inherited £20 000 in gold and gems.If you say that someone or something is a gem, you mean that they are very good, pleasing or useful.You've been an absolute gem - I couldn't have managed without your help.And then he came out with a gem (= clever remark) about the absurdity of the situation.