Slice of agate thinly cut from a nodule or geode and photographed in transmitted light. Agate nodules such as these are formed by the precipitation of silica (SiO2) also known as chalcedony (amorphous silica) in voids and cavities that remain from gas bubbles in the lava after their solidification into volcanic rock. The concentric banding is explained by fact that the silica precipitates intermittently on the periphery of the void from waters percolating in and out of the voids over the geologic eras. Varying impurities or trace elements in the silica account for varying hues and coloration of the precipitate. Actual height is 85 mm.