One of the many sites for fire agate in the State of Chihuahua is located in Aguas Calientes near Ciudad Chihuahua.
Fire agates are showing a game of iridescent shades in red, orange, yellow, and green. This play of colours is caused by refraction of light at thin membrans of iron oxides formed during crystallization of the chalcedony. Uncut fire agates are looking brownish - only the grinding by following carefulyl the layers, always in the growth direction of the chalcedony, brings out the beauty of the stones. A special variation of fire-agates are showing the rare "pinfire". It arises when the membrane is following conoidale forms ( single standing shperulites ) in the chalcedony.
Another origin for fine fire agates is Arizona.