Agate is a variety of chalcedony which is a type of quartz. It is formed when silica-rich groundwater is deposited into the cavities of volcanic rock. Each layer can be a different color depending on the different materials that mix with the groundwater when it is entering the cavity. The layers form within the shape of the cavity which accounts for the unique patterns in each agate. This specimen was collected by Dr. M. J. Dockeray.