Resembling or containing gneiss; having qualities characteristic of gneiss.
From gneiss plus -y suffix. The -y suffix creates informal adjectives meaning 'full of' or 'resembling,' similar to how 'rocky' comes from 'rock.'
Gneissy is the more informal geologist's way of describing something that's clearly gneiss-like, while gneissic sounds more formal—it shows how even scientific language has casual and formal registers!
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