An iridescent cleavage is a cleavage plane inside a gemstone that displays interference colours, typically rainbow-like blues, greens, golds and reds, along its surface. The colours arise from thin-film interference where a partial separation of the cleavage has trapped a thin film of fluid, gas or air between the two faces, or from the inherent optical properties of the structural break itself.

Origin

When a cleavage forms inside a gem, the two faces of the break are typically very smooth and parallel because they follow a crystallographic plane. If the two faces have separated by a small distance comparable to the wavelengths of visible light, the gap traps a thin film. Light reflecting from the top and bottom of this gap interferes, producing the same kind of colour selection that creates oil-slick rainbows on water. The result is a cleavage plane that flashes colour when viewed from the right angle.

Identification and reporting

An iridescent cleavage is a clarity characteristic, plotted in laboratory reports as a feather or cleavage with a notation about the iridescent appearance. In feldspar, fluorite, quartz and topaz, iridescent cleavages are common enough that they appear regularly on grading reports. In some materials the iridescent cleavage is considered enhancing rather than detracting (rainbow quartz, for example, is sold deliberately for the effect), while in others it is a clarity defect.

Distinguishing from intentional features

Some materials produce iridescent flashes as a deliberate feature of their structure (labradorescence in feldspar, for example) rather than as a cleavage characteristic. The distinction is that a structural iridescence is integral to the gem and is visible across the entire surface; an iridescent cleavage is localised to a specific plane that runs through the stone and shows colour only when viewed along that plane.