Fire Agate

Fire agate is a variety of chalcedony, the microcrystalline form of quartz, in which thin layers of iron oxide, limonite or goethite, were deposited between the silica layers as the stone formed. Light reflecting from these internal films interferes to produce flashes of red, orange, gold and green, a thin-film iridescence of the same optical kind as the colour on a soap bubble or an oil slick, not a body colour. The effect is revealed by careful grinding that follows the contours of the layers, so the cutting is unusually skilled and no two stones are alike. It is found only in a limited region of the south-western United States and northern Mexico and was not described until the twentieth century, so it carries no ancient tradition; its metaphysical associations, generally of vitality and protection, belong entirely to the modern crystal trade.

Fire agate's iridescent layers were not recognised and cut as a distinct gem material until the middle of the twentieth century, and the American lapidaries who developed the grinding technique needed to reveal the internal colour without breaking through it kept much of their method informal rather than published, so specific attribution of the discovery is not well documented. The stone remains genuinely rare in the gem trade because its handful of known deposits, in Arizona, New Mexico and the neighbouring Mexican states of Chihuahua and Sonora, are limited, and the skill of cutting it to show colour without loss is still learned largely hand to hand between cutters rather than from a manual.

Fire agate's iridescence is not colour in the ordinary sense. There is no red or green pigment in the stone, and what the eye sees is an interference effect.

Within the chalcedony lie extremely thin, closely spaced layers of iron oxide, usually limonite or goethite, deposited as the silica grew in botryoidal, grape-like masses. Light reflecting from the top of such a layer interferes with light reflecting from the bottom of it. Where the two are in phase, a wavelength is reinforced; where they are out of phase, it cancels. The colour seen therefore depends on the thickness of the film and on the angle of view, and it moves as the stone is turned. Thin-film interference is the same mechanism that colours a soap bubble, an oil slick and the wing of a beetle.

This makes fire agate exceptionally difficult to cut, and the difficulty is the reason good pieces are expensive. The fire lies at a specific depth beneath the surface, following the bumpy contours of the original growth. The cutter must grind down to that layer and no further, following the undulations rather than imposing a shape, because cutting through the layer destroys the effect permanently. Fire agate is therefore carved to a free form dictated by the stone, and two are never alike.

The material occurs in a limited area of the southwestern United States and northern Mexico, in Arizona, California and Chihuahua, associated with Tertiary volcanic rocks. It was not described until the twentieth century and carries no traditional attributions from anywhere.