|Orient & Overtones|
This is one of the most beautiful and unique effects found on a pearl, and the hardest one to explain. This effect is given -in part- by the pearl's nacre thickness, its nacre's spiraled suture lines, its deep luster, surface purity and color, so it is caused by the reflection, the refraction, the interference and diffraction of light upon a pearl. It can be more easily explained as an Iridescence on its surface, allowing the pearl to display subtle color variations as you move it. The more intense the effect is the more desirable the pearl is.
The effect known as orient gets its name from ancient times. The pearls of the Persian Gulf where known as "Oriental Pearls" to the Romans, and thus the name "orient" was tagged to pearls that possessed a unique play of colors on their surface. Ancient Romans obtained pearls from many sources, but most of them came from rivers and lakes in Europe (Scotland, France and Germany), those freshwater pearls rarely had the beautiful orient that saltwater pearls have. Orient is then, a characteristic more common to saltwater pearls.
When this optical effect manifests itself as a deep "halo effect" (mostly in the colors pink/violet and green/blue), the pearl may display as a series of concentric rings (with the color gradually changing to another one) that go from the pearl's diameter and into the center. A pearl with such an effect will never be mistaken as a fake-pearl and it is highly coveted. This effect was once referred to as the "fish eye" effect and is now usually referred to as "overtones". Pearls that were able of displaying it were highly valued.
By the way, the name of this beautiful and singular effect is know as "orient" in light colored pearls, and as "overtones" in dark colored ones. A pearl with Orient or Overtones can never be mistaken as a fake pearl. Whereas -at a glance- artificially colored and bleached pearls can be confused as fakes. Orient /Overtones are impossible to replicate artificially.
Another factor involved in a pearl's orient is secretly hidden from our view. Placing a pearl under a microscope -or using a loupe at 100x- you can see thousands of delicate lines that resemble a "fingerprint". These are always found on the surface of pearls that have orient. Freshwater pearls (and saltwater pearls that have been over-polished) do not have these special markings. (known as "suture lines") These markings allow for additional play of color on the surface of the pearl, causing light diffraction and interference and thus increasing iridescence on a pearl.
So how do these spirals affect a pearl's coloration? Scientists and Naturalists had noticed that iridescence had a common occurrence in nature, it could be seen on pearls and nacreous shells, insect wings, oil spots floating on water and other gems (such as opals). French Gemologist Louis Seraut noticed that if he took wax impressions of insect wings he would be able to replicate their iridescence on the wax itself: it was not a matter of the chemical composition of the wings, but of their appearance. Further study proved that light bounced on these patterns (caused by the circulatory system of the insects) in a very special manner. I hope I can explain this in an easy manner: think of these grooves and spirals as a large canyon system (Mexico's Copper Canyon or Arizona's "Grand Canyon"). Now lets imagine that the canyon's walls are not made of rock but of a reflective material (glass or mother-of-pearl shell) and that we have the sun beaming its rays into the canyon's walls and light starts reflecting on all these uneven structures...what do you believe will happen? Light will bounce off the walls over and over in maddening manner creating such interference that we will have a rainbow-like effect. So, it is in this manner that these little spirals, aided by the other optical effects (reflection, refraction, diffraction) help create Nature's most amazing Gem.