What is the difference between orient and overtone?
In science, one can often solve problems by asking the question... what would water do?
Imagine if you will, looking upon a lake on a sunny day. Like the sky, lakes appear blue even though we know water is not truly blue. This is an example of orient.
In the absence of pigment or emission of light, orient can be best described as the "orientation" of a prism between a light source and our eye.
A variable factor.
We also know water is not perfectly clear. There can be multiple reasons for this. Turbidity, mineral content, algal growth etc. Excepting seasonal variations, it's basically
a fixed factor. This is an example of overtone.
If we were to add ice cubes or some ripples from wind to water, there would be an effect on how our eyes perceive it. Some will reflect, some refract, some occlude. These
random factors are an example of iridescence.
Aragonite (or foliated calcite) are nearly clear crystalline structures. Either may have slightly white or pale brown overtones. The protein that binds aragonite in pearls may contain pigments of any color, even though most species prevail with repetitive colorations.
Therefore, the variable color shift from aragonite crystals is described as orient, while the fixed pigmentation of the protein content is described as overtone. The random combination of both including other contaminants such as calcite, metals etc., the array of stray light is described as iridescence.