Agate's geodes and oco geodes form when gas bubbles are trapped in solidifying lava. Agate is the most common variety of chalcedony which is a form of quartz. Agate is identical with quartz in composition and physical properties.The front of the geode is polished but it isn't the normal high polished "shine" and one area at the top is unpolished rough stone. The base is cut flat. Most agates occur in cavities of eruptive rocks or ancient lavas forming geodes filled with Quartz. Agate is Quartz with impurities causing the many different patterns. Druzy/Drusy agate geode with open pocket of crystals have an opening to the back side. OCO refers to mini or small.