Agate Geode Medium

Agates are a banded variety of chalcedony, which often have layers of intricate banding. Agates are typically formed in igneous rock, where chalcedony gradually fills in cavities left behind by gas trapped within the rock. 
An Agate Geode occurs when the nodule is not completely infilled by the chalcedony, and small quartz crystals are able to form on top of the chalcedony in the space remaining, forming a geode that contains intricate swirls and colour.