Fire agate belongs to the chalcedony family of microcrystalline quartz. These are either solid-colored stones, or have bands or moss-like or dendritic inclusions (agates). The distinctive iridescent colours of fire agate are caused by layers of iron oxide within the quartz. This rainbow effect may be brought by cutting in cabochon.
Fire agate is found in Arizona (USA) and Mexico.
Iris quartz has a similar iridescence, but this is caused by internal cracks.