Nodules in geology are more or less round concretions, compact bodies formed by accretion within a rock. The nodule in this example is Rhyolitic, a volcanic rock type. While it lithifies cracks and voids open up inside, later to become filled with a siliceous precipitate (chalcedony). Cut in half and polished fascinating configurations are displayed, like in this example from West Australia. Actual size is that of a small pear.