Fire Agate is mined in the southwestern part of the US and northern Mexico. This area had volcanoes erupting 24 to 36 million years ago. Water moved in and out over the rock all around the volcanoes. The water was full of iron and silica, which it deposited into the cracks, pits and bubbles forming the water agate. The alternating layers of iron and silica create the iridescent effect.
Fire Agate meaning:
Fire agate is a type of chalcedony that has only been found in particular locations in California, Arizona and New Mexico, and central and northern Mexico.
Fire Agate formed during the Tertiary Period, about 24-36 million years ago when there was tons of volcanic activity in these same regions. While the volcanoes are live, any water that had been present becomes volcanic hot. That water had a lot of silica in it at times and iron oxide in it at other times while the volcanoes were live. The hot water filed any cracks, fissures or holes in the surrounding rock, then it would shake out, then fill again, then shake out and fill again and so on, creating layers of agate. Those alternating layers of silica and iron oxide created a iridescent effect known as the Schiller effect. Same kind of things that happens with the build-up of shells, such as the lining of an abalone shell. That's why Mother-of-Pearl looks iridescent. The alternating layers allows light to pass through and refract.