Here's another entry from Dick Johnson's Encyclopedia of Coin and Medal Terminology.
The reflective colors which appear on certain struck numismatic items. The rainbowlike play of interference colors appears like that of soap bubbles, or mother-of-pearl. It is due to a diffraction of light from closely ribbed or corrugated surfaces, as the striations of a struck metallic piece – flow marks. The iridescence appears to change colors with the point of view or the variation of the angle of the source of illumination. This is highly prized by collectors as the effect disappears rapidly with any wear, handling or tarnish. It is one diagnostic element of a truly uncirculated specimen. This is a form of mint luster and in certain specimens forms a cartwheel effect where the flow marks form spokes from the center outward to the rim. See cartwheel effect, flow marks, mint luster.
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