Fire agate belongs to the chalcedony family of microcrystalline quartzes. These are either solid-colored stones, or have bands or moss-like or dendrite inclusions (agates). The distinctive iridescent colors of fire agate are caused by layers of iron oxide within the quartz. This rainbow effect may be brought out by cutting en cabochon.

Fire agate is found in Arizona (USA) and Mexico.

Iris quartz has a similar iridescence, but this is caused by internal cracks.