Geologists have long been challenged to explain how geodes, those mysterious spherical rocks, are formed. Geodes are a variable phenomenon, which means that they may be created in various different ways. 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.
Each geode is unique in its composition which can only be truly discovered when it is cracked open or cut with a rock saw. The size and formation of the crystals and different shades of color within the crystals make each geode special. The rough exterior of the geode gives no indication of the secrets held within its core. The anticipation never fades for those who curiously collect buckets full of round geodes and eagerly expose the secrets of each individual sphere-shaped rock. The most prized contain rare amethyst crystals or black calcite.
Most geodes have interiors made of either quartz or calcite. Quartz crystals are silicates. Silicates are the most common mineral in the crust of the earth. Over 90% of the minerals present in earth’s crust are silicates. With this said, you can imagine that silicates are a pretty big group with lots of variation in terms of specific chemical composition.
Calcite’s main ingredient is calcium carbonate CaCO2: dissolved calcium and carbon with some of the oxygen from the water thrown in.