WordNet records no synonym and no antonym for geomancy. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
divination by means of signs connected with the earth (as points taken at random or the arrangement of particles thrown down at random or from the configuration of a region and its relation to another)
Synonyms, antonyms and the broader and narrower relations for geomancy come from Princeton University’s WordNet. Full sources and licences.