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nodule - (mineralogy) a small rounded lump of mineral substance (usually harder than the surrounding rock or sediment) mineralogy - the branch of geology that studies minerals: their structure and properties and the ways of distinguishing them.
The term geode is derived from the Greek word Geoides which means earthlike.. A geode is a round rock which contains a hollow cavity lined with crystals. Rocks which are completely filled with small compact crystal formations such as agate, jasper or chalcedony are called nodules. The only difference between a geode and a nodule is that a geode has a hollow cavity, and a nodule is solid. Geodes begin as bubbles in volcanic rock or as animal burrows, tree roots or mud balls in sedimentary rock.