“Looking like a doorway into another dimension, this wonderful crystal of quartz held a secret within. The dark orange brown agate in the centre is surrounded by haloes of iron oxide minerals, probably haematite, and displays the iridescent 3d optical effect known as fire agate.” Loz
Image credit: Fender Minerals
“Coloured a deep chocolate brown and displaying a dazzling iridescent effect somewhat similar to ammolite or opal, fire agates are one of the rarest forms of microcrystalline silica. The colours, that change when the stone is rotated under a light source are due to a phenomenon called thin film interference, something you all see in the sheen of petrol puddles on water.
The layers are composed of alternating agate and iron oxide, and usually occur in this slightly bulgy shape known to mineralogists as botryoidal. They each reflect the light differently, creating waves of interference whose colour depends on the thickness of the layers, which are of similar size to the wavelengths of light. Thicker layers produce reds and greens, and thinner ones the rarer blue and violets.
It is found in the southwest USA, Mexico and Brazil and formed in a hydrothermal geological environment from hot mineralised fluids precipitating silica and iron oxide when encountering changed conditions. These resulted from huge circulating cells of heated waters deep in the Earth interacting with magmas.” Loz
Image credit: Ganoskin Project
via [The Earth Story]