Meaning of “gem” - Learner’s Dictionary

gem

noun [ C ] uk us /dʒem/
STONE

a valuable stone, especially one that has been cut to be used in jewellery

SPECIAL PERSON/THING informal

someone or something that you like very much and think is very special:

She's an absolute gem.

(Definition of “gem” from the Cambridge Learner’s Dictionary © Cambridge University Press)