A nodule was a small rounded mass of matter, typically a mineral, distinct from its surrounding formation.
In 2267, millions of silicon nodules were found on the planet Janus VI. Originally believed by local miners to be rocks of little to no value, they were actually Horta eggs. (TOS: "The Devil in the Dark")
In 2371, the USS Voyager encountered a nucleogenic cloud being which consisted of hydrogen, helium, hydroxyl radicals, and some local dust nodules. (VOY: "The Cloud")