concretion has 8 distinct synonyms spread across 3 of its 4 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.
the formation of stonelike objects within a body organ (e.g., the kidneys)
a hard lump produced by the concretion of mineral salts; found in hollow organs or ducts of the body
“renal calculi can be very painful”
an increase in the density of something
the union of diverse things into one body or form or group; the growing together of parts
Synonyms, antonyms and the broader and narrower relations for concretion come from Princeton University’s WordNet. Full sources and licences.