We don't have synonyms for graywacke yet — it may be a rare, specialized, newly coined, or misspelled word. The closest thesaurus entries we have are grayback, greywacke and grauwacke.
(n) :
Alternative form of greywacke. [(geology) A hard dark sandstone with poorly sorted angular grains of quartz, feldspar, and small rock fragments in a compact, clay-fine matrix.]
(n) :
Alternative form of greywacke. [(geology) A hard dark sandstone with poorly sorted angular grains of quartz, feldspar, and small rock fragments in a compact, clay-fine matrix.]