Iris agate is a rare and prized variety of banded chalcedony in which exceptionally fine, parallel banding produces vivid spectral colours when a thin slice is held against a strong transmitted light source. The phenomenon is purely optical: the closely spaced bands of microcrystalline quartz behave as a natural diffraction grating, splitting white light into its component wavelengths much as a prism or a CD's tracks do. The effect is invisible in the rough or in thicker pieces, and only emerges once the stone is cut into a wafer thin enough that light passes cleanly through the bands.

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Structure and the diffraction effect

The cause of iridescence in iris agate has been understood since the nineteenth century, when Sir David Brewster examined the phenomenon and identified the role of finely spaced lamellae. The bands in iris agate are typically spaced at intervals on the order of one to two thousand lines per millimetre, comparable to a coarse diffraction grating, and consist of alternating microcrystalline silica with subtle compositional or structural differences. When transmitted light strikes these bands at the correct angle, constructive and destructive interference produces concentric rainbow rings or bands of colour that shift as the stone is moved.

Not every banded agate will show the iris effect. The bands must be uniformly fine, regular, and oriented appropriately within the slice. Coarsely banded specimens, however attractive, do not diffract light in this way.

Cutting and presentation

To bring out the colours, lapidaries cut iris agate into thin wafers, often between one and three millimetres thick, and polish both faces to a high lustre. The slice is then displayed against transmitted light, frequently with a dark backing card removed or pierced behind the area of interest. Cabochons of iris agate are uncommon because the diffraction colours are extinguished against an opaque backing; the stone's beauty depends on light passing through.

Some collectors mount thin slices in pendant frames open at the back, so that ambient light can reach the stone, or set them in stained-glass-style window pieces. Free-standing display slices are most common in mineral cabinets.

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Sources

Historically the most celebrated iris agates came from the Brazilian deposits of Rio Grande do Sul, where the volcanic basalts have yielded enormous quantities of agate since the nineteenth century. Mexican fields, particularly in Chihuahua, have produced fine material, and Indian agate from the Deccan traps occasionally yields iris-quality banding. Montana and Oregon localities in the United States contribute smaller quantities. The proportion of iris-grade material from any deposit is small, often under one per cent of the agate recovered.

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Trade considerations

Iris agate is a collector's stone rather than a mainstream jewellery material. Pricing depends on the strength and breadth of the iridescent zone, the regularity of the banding, and the size and clarity of the slice. Specimens with iridescence covering most of the wafer command considerable premiums over those where only a narrow zone shows the effect. Because the phenomenon is intrinsic to the stone's structure, iris agate is not typically treated, although the slices are sometimes lightly oiled or waxed to enhance surface lustre.

The stone is occasionally confused in the trade with fire agate, which produces play of colour from limonite or goethite inclusions rather than diffraction, and with rainbow obsidian, which owes its iridescence to magnetite nanorods. Each of these has a distinct mechanism and appearance, and a careful examination under transmitted versus reflected light readily separates iris agate from its visual relatives.