Fire Agate, a variety of chalcedony, is a semi-precious natural gemstone found only in certain areas of northern Mexico and the southwestern United States.

Approximately 24-36 million years ago these areas were subjected to massive volcanic activity during the Tertiary Period. The fire agates were formed during this period of volcanism when hot water, saturated with silica and iron oxide, repeatedly filled cracks and bubbles in the surrounding rock.

Fire agates have beautiful iridescent rainbow colors, similar to opal.

The vibrant iridescenct rainbow colors found within fire agates, created by the Schiller effect as found in mother-of-pearl, is caused by the alternating silica and iron oxide layers which diffract and allow light to pass and form interference of colors known as fire.

Top image via on Facebook.
Bottom image of raw fire agate prior to refinement via Wikimedia Commons.
Quoted text via Wikipedia.