Grown by both man and mother nature, pearls are a huge subject about which books are constantly being written. Here we are going to try to keep it simple by broadly describing the types or pearls, how they are created and what makes them valuable (or otherwise).
Hydrated organic gemstones - pearls sounds nicer - are commonly produced by bivalve molluscs, especially Pearl oysters and some species of freshwater mussels. Found between the mantle and shell, pearls consist of a protective material called nacre, or Mother of Pearl, which is secreted in reaction to some form or irritation - but not always the fairly mythical 'grain of sand' you might have learned about in school.
The Pearl and its Mother
All waterborne organisms living within shells, called molluscs, have the ability to produce Pearls. However, few molluscs with single shells like gastropods (snail like creatures), do so. Quality Pearls are produced by the bivalve molluscs, which have two-part shells. But even amongst the bi-valve oysters and mussels, the chances of producing a Pearl in nature are a thousand to one.
The popular belief that a Pearl is formed as a result of a grain of sand entering a shell is false. The many layers of nacre, Mother of Pearl, that form a Pearl are made when an organic irritant such as a parasite, or wayward food-particle become trapped within the mollusc, or if the mollusc is injured in some way. You can see why the pearl industry has stuck to the grain of sand story.
Molluscs secrete calcium carbonate, similar to the mineral aragonite, and a binding protein called conchiolin. This nacre, Mother Of Pearl, is the same substance that the animal uses to build its shell. The layers of calcium carbonate and conchiolin settle on whatever it is that has triggered the response and slowly layers of platelets build up to produce the lustre and colour we think of as pearl. Growth rates vary, but some pearls grow at 1mm a year.
Lustre is how we describe the particular diffraction of light from the Pearl's surface combined with the brilliance of its inner reflections. The radiance is the light that is reflected, not just off the surface, but also through all the internal layers. Pearls with a better lustre are achieved when the nacre crystals are uniform and the layers thin and numerous.
Unlike us, molluscs cannot regulate their body temperature, and are susceptible to changes in external conditions. In warm waters of 30 degrees Celsius, the mollusc's metabolism is increased and they grow faster secreting more nacre than a mollusc who lives in colder waters. However, the layers of nacre are thick and not translucent, and the crystal structure is not perfect resulting in a duller less lustrous Pearl. Therefore Pearls are rarely harvested in the hotter summer months.
However, when temperatures go down to 16 degrees Celsius the mollusc's metabolism is slower, and it produces nacre at a slower rate. These nacre layers are thinner and the crystal structure more even resulting in a Pearl with increased translucency and better lustre. So pearl farming is balance between haste and quality.
Pearls appear in a variety of colors from white to black and nearly every other colour in between. Although your first thought might be that these colours are achieved by dyeing, in fact many colours occur naturally. Pearls derive their colors from three sources: genetic make-up of the mollusc, food and trace metals in the water and lastly, to a lesser extent, depth and salt content of the water.
- A rainbow-lipped, or black-lipped oyster tends towards creating darker colours, whereas a white-lipped oyster makes lighter colours.
- Molluscs are filter feeders, consuming algae and diatoms. Many of these carry some form of pigmentation (which can differ between summer and winter) and this tends to find its way into th layers of the conchiolin binding protein. So, red algae = pink pearls etc. In freshwater pearl farming the molluscs are often fed exotic and highly secet recipes containing everything from soya milk to chicken manure. This, amazingly, influences the colours of the pearls which emerge.
- Trace elements acquired from the water, such as metallic ions, also help the Pearl gain its unique colouration. Iron can give a pink color, copper a green to blue hue, magnesium a yellow colouration etc. Pearl farmers know this and the addition of metallic salts to the lakes where pearls grow produce some wonderful deep colours.
- The term 'Orient' or 'Iridescence' is often used in relation to Pearls, Mother Of Pearl as well as to Opal. Orient describes the 'oil on water' rainbow effect across the Pearl's surface, much like the shimmer seen on a soap bubble. This occurs where nacre crystals act like tiny prisms diffracting the light.
Pearl Shape, Size & Surface
Round pearls are incredibly rare in nature. Irregularly shaped Pearls (Baroque Pearls) are much more common. Environmental factors such as the location, water temperature, algae and species of mollusc and length of formation all influence the eventual shape of a pearl.
Very round pearls are a creation of man, not nature. Saltwater pearls can be grown around a nucleus bead.
Freshwater molluscs don't tolerate nuclei being inserted. They form around a small piece of soft tissue implanted from another mollusc. They grow slowly and the resulting unevenness is a sign that they are the genuine product of years of dedicated work by a mussel.
Of course these days you have to be careful as artificial pearls are being produced and given names that can mislead the unwary into thinking they are buying real pearls. How do you tell the difference? The irregular layers in a natural pearl create an uneven surface, because of this it becomes easy to distinguish the natural Pearl from the fakes, which are usually made from ground fish scales or ground shell. Simply rub the Pearl gently across your teeth, if it's natural it will feel slightly coarse to the touch, whereas the fake or artificial Pearl will feel very smooth.
Wild Natural Pearls - Still occur in both salt and freshwater molluscs, but are not harvested to any major degree since the huge growth in cultivation. In nature round pearls are very rare, hence the great value which was placed upon them. Enough large round pearls to make a necklace of wild pearls may be a 1:100,000 event.
Cultured Saltwater Pearls - The three main types of saltwater pearls are Akoya, South Sea and Tahitian, the name being more to do with the species of oyster than their source. Most saltwater cultured pearls are what they call nucleated. Pre-formed beads are inserted into an oyster which secretes a few layers of nacre around the outside surface of the implant before it is removed after six months or more.
Insert a large bead, get a large pearl (actually, not quite that simple as nucleation is a technically difficult procedure). The point here being that many saltwater cultured pearls are very round and the layer of nacre on them can be quite minimal. A good pearl and a poor pearl can be impossible to tell apart without x-ray equipment.
Cultured Freshwater Pearls - Although the pearls look virtually identical to their saltwater cousins, with the same excellent lustre, the differences in cultivation are enormous.
The first difference is that freshwater pearls are grown in a variety of mussel, not an oyster. Hyriopsis Cumingii and its various hybrids is a brute of a mollusc and only distantly related to either the oyster or the moule you might enjoy with frite.
Unlike cultured saltwater pearls, freshwater pearls are not bead-nucleated and are therefore typically less round. Instead of using a bead, a small square of mantle tissue is cut from a donor mussel's inner lining and inserted into the lining of the harvesting mussel to instigate pearl formation.
This cultivation technique makes freshwater pearls which are purely composed of nacre. Nacre is the mother of pearl substance that gives pearls their distinguishing glow. It also is one of the main value factors for grading all pearl types. Because cultured saltwater pearls are bead nucleated, their nacre thickness ranges between 0.2 mm and 4.0mm and they have usually spent less than 1 year in the oyster.
A 12mm freshwater pearl growing at approx 1mm per year is a venerable creature indeed and 100% nacre.
Only two percent of all freshwater pearls are round or near round. A 10mm+ round pearl might be a 1:10,000 event. Though this number seems quite small, the abundance of freshwater pearls is much greater than other pearl types. Each freshwater mussel will produce up to 40 pearls, whereas one saltwater mollusc usually only produces one or two pearls.
The most common shape in freshwater pearls is oval or button shaped (sixty percent). Thirty-eight percent are Baroque and Semi-baroque. Almost all Freshwater pearls come from China. Since the 1990s, Freshwater pearl quality has steadily increased. Though you rarely have a perfectly round Freshwater pearl, the likelihood of finding a quality near round pearl is much greater today.
Akoya Pearl Oyster - Pinctada Fucata
Japan is famous for its Akoya Pearl producing oysters, but this has only been the case since the 1920s. Before this time Japanese divers placed little value on Pearls, discarding them and keeping the Mother Of Pearl, which was used as decorative inlay in jewelry and household ornamentation. By the 1950s, with new culturing techniques, Akoya Pearl had won widespread popularity and is now exported worldwide.
Mabe Pearl Oyster - Pteria Penguin and Pteria Sterna
These two species of thin-shelled oysters are the most common sources of Mabe blister Pearls. Mabe Pearls are artificially induced or cultured, they form when a mold is inserted between the oyster's shell, this causes the oyster to produce layers of nacre which eventually cover the mould, the mould is then cut from the shell. These oysters occur naturally in the Red Sea, Indian Ocean, and the western Pacific.
Freshwater Pearl Mussel - Hyriopsis Cumingii
A close relative of the Biwa pearl from Japan, HC is now the major pearl producing mollusc in the world. Unlike oysters, freshwater pearls have to grow to full size naturally and cannot be formed around a nucleus inserted into the shell.
Wearing Pearl Jewellery
Pearls can be elegant and formal, or fun and informal. And the nice thing about pearls is that you can find them to go with every skin tone.
The Collier or Collar Pearl Strand Length: 12 to13 inches. The Collier is made up of more than one strand of uniform Pearls that fit cosily around the base of the neck. A popular 'Coming-of-Age' gift.
The Choker Pearl Strand Length: 14 to16 inches. The Choker is made up of one strand with large uniform sized Pearls that sits on the collarbone.
The Princess Pearl Strand Length: 17 to19 inches. One of the most popular lengths the princess is well suited to crew and high necklines, but can equally add a frame to a plunging neckline.
The Matinee Pearl Strand Length: 20 to 24 inches. The Matinee, an unrestricting length that looks great with casual or business wear, be it a sweater or the classic white blouse.
The Opera Pearl Strand Length: 28 to 36 inches. The Opera Strand falls to the breastbone and usually consists of pearls larger than 8mm.
The Sautoir or Rope Pearl Strand Length: 40 inches and over. The preferred length of necklace by the French haute-couture designer Coco Chanel. It's a lot of pearl, but very, very flexible in how and where it can be worn.