Fire agate, a variety of chalcedony with an ancient and fiery beginning, is an all natural gemstone found only in certain areas of Arizona, northern Mexico, and other parts of 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 vulcanism when hot water, saturated with silica and iron oxide, repeatedly filled cracks and bubbles in the surrounding rock, often forming a botryoidal (grape-like) growth.
Fire agate gemstones have beautiful iridescent rainbow colors, similar to opal, with a measurement of hardness on the MOHS scale of between 6.5 to 7 which helps prevent cracking and scratching. The colors found within fire agates, created by the Schiller effect as found in mother-of-pearl and fire opal, is caused by the alternating silica and iron oxide layers which diffract and allow light to pass which forms the interference of colors known as fire. Besides the thin limonite and/or geothite iron oxide layers there is no actual objects inside the gems, the visual effects arise from light interference within the microstructure crystal layering of the gem.