Jewelry by Galatea
Luster is THE key attribute of pearls that elevate them into the "precious gemstones" category.
Luster is created when light strikes the surface of the pearl and rebounds back towards the observer, giving the viewer a bright, shiny and reflective pearl surface to admire.
Luster is also what gives pearls the soft glow that appears to come from inside the pearl. This occurs because light waves penetrate the surface layers of nacre, travel to the pearl's interior and then bounce back through the layers of crystal to the viewer, imbuing pearls with a gorgeous inner glow.
So what creates that gorgeous, amazing, phenomenal luster??
Nacre. Nacre is quite simply, the crystalline building blocks that make up the foundations of a pearl's luster.
Nacre platelets are microscopic, roughly hexagonal-shaped crystals made up of semi-transparent calcium-carbonate (CaCo3) "glued" together with conchiolin. The mollusk secretes nacre in continuous, tightly compacted layers over an irritant or parasite that gets inside its shell, coating and smoothing it over with thousands of layers of crystalline nacre. The resulting structure can be simultaneously described as onion-like and also like a brick wall in its construction.
The electron microscope photograph above from Science Mag shows what this interlocking, layered construction actually looks like. To get a sense of how this arrangement transmits light and iridescence, check out the diagram I created below. The iridescence is what creates the visual phenomenon of Orient, but I'm saving that for another post. ;)
Different pearl types display varying degrees of Luster; while they all have some aspects in common, each pearl type "plays" with light in its own unique way. Let's examine each major pearl type and see how that plays out in real life, and what you can use when deciding which pearl type to buy.
Akoya pearls are farmed in the cold waters of the Pacific Ocean off the coast of Japan (primarily! There are other locations where Pinctada fucata martensii make their homes ...). They are famous for their "mirror-like" or "ball-bearing" luster, which is very sharp, highly reflective and has a high rate of light return, making these pearls beautifully bright.
The oysters are seeded during the Spring to Summer months, and harvested later in Fall to Winter when the colder waters temperatures act to slow the oyster's metabolism down. Farmers believe this causes the mollusk to secrete its final nacre layers more slowly and compactly, imparting the pearls with their bright, hard shine. This compact layers creates a crystalline surface that causes light to strike the pearl's surface and bounce back at a faster rate, giving Akoya pearls their trademark "mirror-like" luster.
Freshwater pearls from China are the only solid-nacre cultured pearls available on the market today. These pearls are "tissue-nucleated" meaning that instead of a round mother-of-pearl bead nuclei as its "seed", a small 1.0mm piece of donor mantle tissue is inserted into the mollusk to jump-start the pearl creation process. As the mussel coats the irritant with nacre, a pearl begins to form. Slowly, the organic tissue inside degrades, leaving us with a solid-crystalline pearl! So what does that mean in regards to the type of luster these pearls display?
Light takes longer to travel through the layers before returning to the viewer, which creates a softer, satiny luster for this pearl type. Objects and people reflected in the surfaces will not be completely recognizable, and you may not be able to see certain facial features such as your eyes or your smile. Metallic Freshwater pearls are the exception to this rule, and mimic the Akoya with their glossy, metallic luster but they are rare, accounting for just 1 in 3,000 pearls.
Freshwater pearls also often display that beautiful inner glow unique to pearls; the finest Freshwater pearls can look like milk glass ... nearly transparent in certain lighting conditions and with careful observation. In my humble opinion, Freshwater pearls are the unsung heroes of the pearl world.
Tahitian pearls are the most famous naturally-black colored cultured pearls in the world, grown in the warm, tropical waters of the French Polynesian Islands. They are also one of the larger cultured pearl types, and are known for having very thick nacre layers. This thick layering of nacre helps create their beautifully saturated colors, but as with Freshwater pearls, the thicker nacre slows light transmission returning from the interior of the pearl layers to the viewer, resulting in a slightly softer luster than that of Japanese Akoya pearls.
Very high quality Tahitians like our AAAA and Gem Quality pearls can come close to the Akoya's ball-bearing shine, however those pearls are most often seen in the smaller sizes (7.0-8.0mm through 9.0-10.0mm) which are produced by younger oysters with a higher metabolism capable of producing numerous layers of nacre quickly and compactly. Older oysters produce pearls in the largest size ranges (12.0-13.0mm through 15.0-16.0mm), but often the luster is visibly softer and more "blurry", and the color/overtone range of the pearls tends to be more Medium-Dark Charcoal Grey with Silver.
Both White and Golden South Sea pearls are the largest of all cultured pearl types, and are known for having very thick nacre layers - around 2.0-4.0mm thick on average. Like Tahitian pearls, their thick layers of nacre helps create their beautiful colors of Silvery-White and Gold, but also slows light transmission returning from the interior of the pearl layers to the viewer, giving the pearls their trademark satiny look and soft inner glow.
Rarely, very high quality South Sea pearls, Gem Quality grades, can come close to the Akoya's mirror-like luster, but those pearls are most often seen in the smaller sizes (7.0-8.0mm through 9.0-10.0mm). The larger sizes, 13.0mm and up typically have luster that is visibly softer and more diffused in appearance.
These pearls are farmed in warm, tropical waters of the Philippine Islands and Western Australia, and so the oysters layer nacre more quickly, with a slightly looser crystalline arrangement than the Akoya.
Their extremely thick nacre layers average between 2.0-3.0mm thick, so while the sharpness of the luster may not be "mirror-like", South Sea pearls often possess a beautiful, ethereal glow, and long-lasting durability.
That about does it for this edition - I hope everyone found it helpful and educational. For more information on the science between pearl nacre, luster and the visual phenomenon of Orient, I highly recommend checking out my full article on Pearl Nacre below.
Until next time!
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