Mr. Webster defines iridescence as In the coin realm, the words and used todescribe a coin's appearance are almost always accompanied by the words, , hues, , , , ,, , .

The vocabulary term recited in last week's E-Sylum states that iridescence ofnumismatic items is due to a diffraction of light from flow marks, and that it is aform of mint luster. If that indeed were the case, toning enthusiasts would besearching freshly produced coins directly from the mint for rainbow beauties. Ofcourse, that is not the case. While flow lines help to create the appearance ofluster, even the highly desirable cartwheel effect, it is not flow lines, but anotherphenomenon that is the usual cause of iridescence on a coin.

Rather than using soap bubbles or nacre as an analogy, as the vocabulary termdoes, conjuring an oil spill on a flat-water surface creates a better image in one'smind of how colors may appear on a coin's surface (bubbles and pearls are round,yet water (a coin) is flat).

The thinnest layer of oil atop water can result in an eye-full of rainbow colors.How does this happen?

While some light waves bounce off the top surfaces of the oil, other light wavesare redirected as they pass from one medium to another (refraction), and bounceoff the coin surface below. The upper layer of oil creates one surface, while thewater level creates a second and lower surface. The combination of bouncedlight off the oil, and refracted light bounced off the water level converge on thehuman eye to create colors via what is known as thin-film interference.

Thin-film interference is a natural phenomenon in which light waves reflected bythe upper and lower boundaries of a thin film interfere with one another, eitherenhancing or reducing the reflected light.

Think of the water level as the coin's surface, and the oil level equivalent totoning, or the level of interaction of a coin's surface with its environment overtime (patina). As the nature, length, and depth of the environmental interactionchanges, so does the level of thin film interference, thus altering the colors thatthe human eye perceives on a coin's surface.

Iridescence does not necessarily disappear rapidly due to tarnish, as thevocabulary term states. It is actually created from, and may actually be enhancedas the level of toning advances (you say tarnish, I say toning). Of course, as patina(toning) advances on a coin, colors may become less eye appealing or cancel outaltogether. Not all toning is attractive, as Mother Nature can be very fickle andshe will do what she pleases.

Not to complicate matters, but it is actually a misnomer to refer to toning on acoin as toned. Although a rainbow is created by refraction, as is thin-film interference to a degree, they are not the same thing.

The vocabulary term for iridescence conflates how the human eye sees mint(cartwheel) luster with how it sees toning on a coin. Hopefully this helps toseparate the concepts of luster and toning.

To read the earlier E-Sylum article, see:
VOCABULARY TERM: IRIDESCENCE(https://www.coinbooks.org/v25/esylum_v25n23a15.html)