Optical
Effects on a Pearl:Optical
Effects on a Pearl:Most people find it impossible to explain certain optical effects displayed on a pearl's surface. Some people even confuse Orient with Luster and vice versa. Here we will try to explain and demystify these elusive terms, as well as give you an additional insight into things that are important to the pearl and its owner.
Most people state that a pearl's luster is the way it shines or reflects light. Pearls are not "shinny beads" they don't simply shine like polished rocks, shoes nor floors. Instead, pearls Glow. The soft glow of the pearl is the real effect of luster and orient in it. A pearl's luster is really the combination of two very similar effects: surface luster and deep luster.
But to explain these effects we need to understand how a layer of nacre works. Nacre is very thin and translucent, but also reflective. When light hits the surface of a pearl some of it will be bounced off the surface of the pearl (like in a mirror) but some of the light will also go through this layer and into the next one (like glass). The process then repeats itself millions of times, over and over. So each layer of nacre is bouncing light and deviating it, thus creating the effect of a soft, silky glow. This is deep luster (figure 2).
Surface luster (figure 1) happens when the outer coatings of the pearl are very thin and the crystals are well organized, so they are able to reflect light. You can measure superficial luster by comparing it to the way a mirror reflects objects: the sharper your image on the pearl's surface, the more lustrous it is. This effect can be increased with polishing, but it can be affected by UV light, heat, and chemical substances, since pearls are delicate creations.
And, as you can deduct from the picture above, a thinner pearl (left) has fewer coatings of nacre than a thickly coated pearl, so the effect of "deep luster" will not appear on thinly covered pearls. The thickly covered pearl has many more layers on which the light can play.
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. The Romans obtained pearls from
many sources, but most of them came from rivers and lakes in
Europe (Scotland, France and Germany), those freshwater pearls
never had the beautiful orient that saltwater pearls had. Orient is then, a
characteristic common to saltwater pearls.
The play of colors on the surface of a pearl is a combination of factors. The pearl must have a minimum of luster, of nacre thickness and skin purity in order to have Orient. The effect manifests itself as a play of colors (mostly pink/violet and green/blue), and the stronger this effect is the more intense the colors are and they 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. Sometimes this effect is hard to describe, but a picture can help.
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 can never be mistaken
as a fake. Whereas -at a glance- artificially colored and
bleached pearls can always be confused as fakes. Orient is
impossible to replicate.
Another factor involved in a pearl's orient is secretly hidden from our view. Putting 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. These markings allow for additional play of color on the surface of the pearl, thus helping in the appearance of iridescence (also known as chroma).
Pearls are the creation of a living organism. Environmental factors play an important role in the formation of a pearl.
Pearl oysters are incapable of regulating their body temperature, so it fluctuates in the same way their surrounding watery environment does. At a high temperature (30 Celsius) pearl oysters have a very high metabolism, so their activity is increased: they grow faster, and secrete nacre very fast as well. Pearls grow fast in summertime. But there is a downside to this, nacre coatings are thicker (less translucent) and the crystal arrangement (Aragonite) is far from being perfect. Pearls harvested in summer tend to be duller or less lustrous.
On the other hand, when water temperatures are low (16 Celsius) the oyster's metabolism is at its lowest, so it secretes nacre at a very low rate. The nacre layers are very thin (translucent, good for luster) and Aragonite crystal arrangement is done perfectly. Pearls harvested in wintertime tend to be very lustrous.
Pearls must be grown in clean waters. Good quality pearls can only grow inside healthy oysters living in a healthy environment. Pollution in the water can weaken oysters and kill them (as in Japan), inhibit the secretion of a good coating of nacre, as well as introduce pollutants into a pearl (think of Yellow colored pearls) that inhibit the appearance of orient.
A pearl's color comes from three main sources: the pearl oyster's genetic make-up, available food and trace metallic particles. Other factors that are involved are culture depth and salinity, but they are less important.
A pearl oyster's genetic makeup is partly responsible for the color of the pearls one can obtain. A black-lipped and a rainbow-lipped oyster can produce a wide array of dark colored pearls, a white-lipped oyster can -usually- only produce lighter colored pearls.
You still have food intake to consider: Pearl oyster's are filter-feeders, meaning that they are able to extract minuscule particles from the water. Many of these particles consist of phytoplankton, microscopic one-celled algae. Depending on the season, several species of phytoplankton are available for the oyster. In the Gulf of California, cyanophites (blue-green algae) are more common in winter, whereas diatoms (that have a golden silicate covering) are more common in summertime. They provide the oyster with substances they can use (unwillingly) to "color" their shell and nacre.
Other trace substances can be acquired directly from the seawater, such as many metallic ions, that help the pearl gather its unique coloration. Iron (Fe) can give a pink/violet color, Copper (Cu) for green/blue, Magnesium (Mg) for yellow, and so forth.
Pearls are formed of many inorganic and organic substances, that joined together make up Nacre. Pearls are not a pure substance (like a diamond, made up of Carbon (C) and trace elements), but they are made up from a compound that embodies purity.
Chemically speaking, pearls
are made of Calcium Carbonate, Protein and Water. Their main
source of Calcium Carbonate is in the way of Aragonite,
which has flat hexagonal shaped crystals. These crystals are bound by means of a protein known as Conchiolin,
and this protein has a fair amount of water in it. Trapped
between the crystals and the protein are several trace elements,
mostly metallic (color giving) ions.
So, as you have seen, the optical effects on a pearl (luster, orient and iridescence) are part of an intricate system that involves chemical properties, the cleanliness of the surface, the way nacre layers are deposited, the species of pearl-producing bivalve involved, the waters they are grown, etc.
Copyright 1996-2007 Perlas Únicas S.A. de C.V.
Last Updated 05/2007
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