Agate is the banded form of the mineral Chalcedony, which is a microcrystalline variety of Quartz.
Agate is the most varied and popular type of Chalcedony, having many varieties on its own.
Agate usually forms in rounded nodules or knobs, recommended to be sliced open to bring out the internal pattern hidden in the stone.
The formation of Agate is most often from the deposition of layers of silica filling voids in volcanic vesicles or other cavities.
The layers form in stages with some of the new layers providing an alternating color.
Since the cavities are irregularly and uniquely shaped, each Agate forms its pattern based on the original cavity shape.
When a cavity filled, it forms a solid mass of Agate, but often it is only partially filled, leaving a hollow void which usually has crystalline Quartz growths on its innermost layer. The cause of Agate forming the outer lining of most geodes.