Fire agate is a layered stone. The layers are small enough that light entering them forms interference colors known as "fire". The gem is formed when hot water saturated with colloidal silica and iron oxide invades cavities in country rock and begins to cool. Chalcedony with iron oxide begins to grow on any available surface (the iron oxide gives the basic brown color to the gem). As the solutions began to precipitate and grow layers of silica and iron oxide would be deposited depending on the relative level of those elements in solution and underlying conditions. These alternating silica and iron oxide layers (Schiller layers) cause the brilliant fire in the gem. As iron oxide ran out in the solution colorless chalcedony continued to grow.
Hand polished on both sides. From Mexico.