the agates are created by heat and pressure filling cavities in igneous rock with layers of silica over time. the first layer of silica would line the inside of the cavity and crystallize. then, during another geologic event, a different set of minerals and silica would form on top of these layers, essentially forming the layers from the outside in, explaining why some agates have cavities in the center. these agates are incomplete, and have not been fully filled with silica.