Gem of the Month

Each month this section will feature either a topic of interest to gem lovers or one special gemstone with background on the material and its value.

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October, 2005

Natural Pearls

Prior to the 19th century, when they were superseded in price by diamonds, natural pearls had, throughout history, been valued above all other gems. Although their beauty, and the fact that they come out of the mollusk ready to use, were important factors, it was sheer rarity that drove their value to the highest levels. The formation of a large, beautiful and perfect natural pearl is an event so unlikely in Nature that only those at the pinnacle of wealth and power in a society were able to own them. Depending on the species between 1/1000 to 1/500,000 mollusks will form pearls during their lifespan, and the vast majority of those formed will be small, off-color or flawed.

Nacreous Pearls

True pearls are referred to as "nacreous pearls" due to their composition. Such a pearl is formed when a small foreign object makes its way into the soft body of a filter-feeding or grazing mollusk, and cannot be expelled. The irritant is sometimes a stray bit of shell or bone, but more often a parasite. Layers of calcium carbonate crystals and protein are secreted to slowly cover all parts of the object which, then, becomes a pearl. If it grows entirely within the body of the animal it will be three-dimensional "cyst pearl", if it grows attached to the shell, it will be a "blister pearl".

We can perhaps imagine the awe and mystery that these objects held for the early peoples who found and cherished them, and it is no wonder that mythic and mystical explanations for their formation abounded. Picture harvesting the creature shown below (not all pristine and clean, as shown), but covered with mud and sea weed and inside its greyish, lumpy and slimy interior, finding the iridescent object in the adjacent photo.

[Abalone shell, abalone pearl jewelry: Images courtesy of www.allnaturalpearls.com]

The beauty of a nacreous pearl comes from a combination of its shape, color, and surface reflection (luster). In the best specimens these features are heightened by a surface iridescence called "orient". The shape of a pearl will largely be determined by chance (the shape of the irritant), and its anatomical placement in the animal. The body color will vary with the species of mollusk, which will generally make pearls in shades similar to that of their shell lining. The iridescence and/or luster of a pearl will be a consequence of the perfection and thickness of the nacre layers, of which the onion-like pearl is made.

Nacre (NAY-ker) is made up of plate-like hexagonal crystals of translucent aragonite (a form of calcium carbonate), conchiolin protien (konk-KY-oh-lin) and water. Each crystal is very thin and they are layed down rather like bricks in staggered courses with the proteinaceous "motar" between. Light, reflecting and diffracting from the uneven surface, and the thin inner layers creates the lovely effects of luster and orient.

[Magnified surface of a pearl with its overlapping layers of nacre: Image courtesy of Joe Mirsky]

Pearls are made by both salt and freshwater bivalve (two shell: oysters and mussels) and univalve (one shell: snail-like) mollusks. What we call saltwater pearl "oysters", are not closely related to the edible varieties of oyster. So, that plate of oysters you might enjoy for lunch, must remain a gastronomic delight alone, as no pearl will be found in it. If you are a freshwater mussel or saltwater abalone fan, however, your chance of finding a pearl, though slim, exists, as long as you eat them raw (cooking destroys pearls).

What's in a name?

In today's world, the word "pearl" means "cultured pearl" to almost everyone. Technically, it's illegal to sell or advertise cultured pearls, as pearls, without using the adjective "cultured", but no one really gets very excited about enforcing it. In the present day, natural pearls, due to overfishing and pollution, are even rarer than they were in historical times. Their admirers are not so much the wealthiest among us, as those who for philosophical, spiritual/religious, or aesthetic reasons seek out these rarities. This shift in the position of natural pearls from status symbol to quasi-cult objects, has been the result of the spectacular success of pearl culturing. With its technical beginnings around 1890 and large scale production in force by the early 1920's pearl culturing has made this gem one that is obtainable by virtually anyone in a variety of qualities and prices.

How the pearl marketplace has changed, then:

[Circa 1890 2-3.5 mm, natural, saltwater pearl brooch, circa 1920 2.5 - 4 mm, natural saltwater 16" pearl necklace]

Natural Pearls: Expensive then, expensive now!
And now:

[Circa 1960 4 mm, cultured saltwater pearl brooch (fairly expensive when new), 2005, 18" 7.5 mm, freshwater cultured pearl necklace (very inexpensive)]

Cultured Pearls: Size is increasing and prices are dropping!

In looking at the photos above we can begin to see why cultured pearls have taken over the world. Natural pearls are generally small, and vary in shape and color, cultured pearls, especially in today's market, are uniform in size, shape, and color and can be huge! It might take an oyster in Nature six or seven years to make a 4 mm pearl, and only 1-2 out of a hundred of them would be round. Furthermore, colors are not uniform.

In today's market natural pearls are available through current small scale (legal) harvesting, and also as vintage and antique specimens or jewelry.

Examples of natural, nacreous pearls:

[A contemporary suite of saltwater pearls from the "Rainbow-lipped" oyster: Image courtesy of www.allnaturalpearls.com, Victorian Era seedpearl bride's necklace, Art Deco Era saltwater pearl necklace, contemporary suite of abalone pearls: Image courtesy of www.allnaturalpearls.com, circa 1905 gentleman's saltwater pearl stickpin]

Are they really natural?

If you're in the market for antique jewelry with natural pearls you need to be aware that imitation pearls have a long history and were sometimes used in surprisingly "upscale" jewelry pieces. The "tooth-test" is generally helpful (although not sanitary). The surface of a nacreous pearl (natural or cultured) will be slightly gritty. This microscopic roughness can be detected by rubbing the pearl, gently, across the edges of your front teeth. Pearl simulants (usually made of glass or shell) have smooth surfaces and don't feel rough. Also if the pearls in your antique piece are uniform in size, luster and color your "fraud antennae" should be on full alert.

Non-nacreous "pearls"

Technically, the product of a mollusk that is not made of nacre is not a pearl, but a "calcareous concretion". Having said that, I'll point out that they run the gamut from chalky marble like products such as found in edible oysters with no gem value, to some of the most highly valued and rare gems in the world. For the purposes of this essay, I will call these beautiful and valuable ones, "pearls".

There are three of note: Conch pearls (pronounced "konk"), scallop pearls, and melo melo pearls. Each of these is made of calcium carbonate but primarily in the form of calcite rather than aragonite, and with different structural characteristics and protein proportions than their nacreous cousins.

Conch pearls are products of a large marine snail, the queen conch, It is native to the Caribbean and, until it was fished to near extinction, was found abundantly in the waters of the Florida Keys. Ranging in color from white to vibrant pink, the pearls are usually small (8 mm is large) and ovoid. you can see in the picture below and to the right, the highly desirable "flame structure" chatoyance that the best specimens have. (Conch pearl lovers are not to blame for the decimation of the Florida population, as they are basically just a rare by-product of the hunt for this mollusk: the meat of the conch is a delicacy, its pink shell lining is used in jewelry, especially cameos and the shell itself is a tourist object.)

[Strombus gigas, the Queen conch, with typical pearls: Image courtesy of www.sunlion.com, three top quality conch pearls showing "flame structure": Image courtesy of Aires Jewelers, an attractive and protective contemporary 14k setting for a small conch pearl: Image courtesy of www.faisongems.com]

Scallop pearls

The newest type of natural pearl available to collectors is the scallop pearl. It is found in a marine bivalve scallop that is native to the coast of Baja California, and is just beginning to be harvested. Highly variable in size and shape, they have mosaic-like patterns and cream to salmon or mauve colors with a semi-metallic to chatoyant sheen.

[Scallop pearls: Images courtesy of www.allnaturalpearls.com]

Melo melo pearls:

By far, the hardest to obtain pearls on Earth are those of the marine "baler" snail, found in the Indo-Pacific region: round, smooth and sometimes quite large, I recently held one in my hand that was the size of a large gumball and nearly dropped it when the price of $50,000 was quoted to me. The colors and flame structure are similar to those of the conch pearl.

[Image courtesy of the Latendresse family]

To this date, none of the non-nacreous pearls have been successfully cultured, and each has a unique structure which makes its difficult to fake, so that there is little worry about synthetics and simulants.

Value

It is difficut to talk, except in generalities, about value for gemstones as rare and variable as natural pearls. The nacreous ones increase exponentially in value with size but luster, color and shape are important as well. With the non-nacreous pearls, color and quality of the any surface pattern or chatoyance are probably more important than size.
Care

All pearls need gentle care. They are soft, fragile, and are sensitive to chemicals, especially acids. All the cleaning they ever need, is wiping with a damp cloth after each wearing, and they should be stored away from other gems, preferably in a cloth bag or their own case. Non-nacreous pearls should not be exposed to bright light for extended periods as the organic pigments which give them their colors can fade. Pearl strands that are worn frequently should be restrung every few years.


Gemological Data

Varies with species


Stones Currently Available
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Go to: Homepage -- what's new in faceted gems -- what's new in designer cabochons and gem carvings -- gem of the month -- gem of the month archive -- birthstone of the month -- key to all the codes used on the ACS site -- definitions of terms used on the ACS site -- how to order -- about ACS -- about the ACS cutters -- settings for these gems --faceting information -- purchase UltraTec equipment