diamond has 7 distinct synonyms spread across 4 of its 6 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.
Synonyms, antonyms and the broader and narrower relations for diamond come from Princeton University’s WordNet. Full sources and licences.
a parallelogram with four equal sides; an oblique-angled equilateral parallelogram
a playing card in the minor suit that has one or more red rhombuses on it
“he led a small diamond”
the area of a baseball field that is enclosed by 3 bases and home plate