gem has 6 distinct synonyms spread across 5 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a crystalline rock that can be cut and polished for jewelry
“he had the gem set in a ring for his wife”
a sweet quick bread baked in a cup-shaped pan
a precious or semiprecious stone incorporated into a piece of jewelry
Synonyms, antonyms and the broader and narrower relations for gem come from Princeton University’s WordNet. Full sources and licences.