A pearl is a reaction to an irritant within a mollusk. Pearls are formed when the mollusk secretes thousands of very thin concentric layers of nacre (a secretion of calcium carbonate (aragonite and conchyolin) in a matrix that eventually coats an irritant, either man made or natural. The thin circumferential lamellae of nacre intersect the external surface of the pearl to create a 'thumbprint pattern' that characterises the surface of nacre.
Pearls form inside a mollusk, which is an invertebrate with a soft body often protected by a shell such as clam, oyster and mussel. Any mollusk is capable of producing a pearl, although only those molluscs that have shells lined with nacre produce pearls that are used in the Jewellery industry.
The pearl is unique among all other gemstones in the world. It is the only gem that is grown inside a living organism.
There are three main pearl types used in Jewellery:
• South Sea (White, Golden and Tahitian)
Each type of pearl is produced by a different species of oyster, and each oyster lives in a different region of the world under very specific climatic conditions.
Determining Pearl Quality
Pearls are unique among gemstones in that they are natural, organic products of living creatures, so they have different grading standards. Seven factors determine the quality, value, and beauty of pearls: size, shape, colour, luster, surface quality, nacre quality and matching. A pearl's ultimate size and quality can depend on several variables - the size and health of the mollusk that produced the pearl, the size of the nucleus, and the amount of time the mollusk spent underwater adding layers of nacre to build the pearl. Other factors also include the climate and the nutrient conditions of the mollusk's growth environment.
Pearls are measured by their diameter in millimeters. Tiny seed pearls can be smaller than 1 mm, while South Sea pearls as large as 20 mm have been found. If all other quality factors are equal, the size of a pearl will determine its value. Only a 1 millimeter increase in pearl size is a substantial jump in both appearance and value. The importance of a pearl's size to its ultimate value depends on the pearl type. If a high quality pearl is larger than usual for its type, it will be worth more than smaller pearls of the same type. The average pearl sold today is between 6.5mm and 11.0mm.
As pearls are natural organic gemstones, they can occur in a wide variety of shapes, many of which are quite unique and interesting. The round pearls you most commonly see are by no means the only shape in which pearls are found!
Indeed, perfectly round pearls are actually quite rare. This is because the eventual shape of the pearl is determined by a number of highly variable factors which occur inside the oyster as the pearl is developing. For example, the pearl often assumes the same shape as its nucleus (the irritant which was placed inside the oyster to initiate the formation of the pearl which guides the process of the pearl shape). If the nucleus is not perfectly round, the resulting pearl is likely to reflect this irregularity.
In addition, the pearl's positioning within the oyster also plays a role in determining its shape. If the pearl develops against the shell, for example, it will become more flattened on that side. Some pearls, for example, develop with one or more grooves or rings encircling them. These pearls are known as ringed or circled. The primary shape of a pearl is therefore described as circled round, or ringed oval. These circles or rings may occur if a constant pressure is placed against the pearl as it is developing within the oyster.
Pearl experts generally divide pearl shapes into three broad categories, based on their overall characteristics:
• Spherical shapes are perfectly round or nearly round. They are the classic pearl shape that is most familiar.
• Symmetrical shapes are balanced and regular. If you sliced this pearl in half, each half would be a mirror-image of the other half.
• Baroque shapes are irregular or abstract. They are non-symmetrical in nature.
Within these three broad categories, pearls can be classified into seven basic shapes:
A pearls colour contains three basic components - hue, the colour's first impression; tone, its lightness or darkness, and saturation, its strength or intensity. Pearl colours tend to be muted displaying a broad range of subtle hues. A Pearl's colour can have three main characteristics.
• Body-colour is the dominate overall body colour of the pearl.
• Overtone, one or more translucent colours that appear over a pearl's body-colour.
• Orient - iridescent, rainbow colours shimmering on or just below a pearl's surface.
Pearls come in a variety of colours, from white to black and every shade in between. All pearls display body-colour but not all pearls will display overtones or orient. It is important to distinguish between colour and overtone. For example, some naturally occurring colours are white, champagne, aqua, green, golden, and black. Within each colour category, there are a number of common overtones, or subtle variations in the surface iridescence. Overtones are translucent colours which sometimes appear over top of the pearl's main body-colour. These overtones tend to alter the body-colour somewhat, as well as adding depth and glow. A pearl may be white with rose overtones, for example.
Some pearls have no overtones at all while some display an orient that show a rainbow iridescence which is an incredibly rare factor only seen in approximately .01% of pearls produced today. The term orient refers to the shimmering, iridescent colours which appear to move and glitter when the pearl is turned. This phenomenon is caused by the way in which light is reflected through the various thin layers of nacre which make up the pearl.
Lustre is the most important of all the value factors when considering the beauty of a pearl. Luster is more than a combination of surface brilliance and a deep glow. It is caused by light traveling down through the translucent layers of nacre and reflecting back to the eye from deep within the pearl.
The translucence and arrangement of the overlapping nacre plates are more important influences on whether a pearl will have high lustre. With a good quality pearl you should be able to feel the difference and see your reflection on the surface. Growth conditions can influence the lustre of a pearl, for example the quicker the nacre is produced, the less translucent it is likely to be. As a result the nacre may be thick but not lustrous.
There are four categories of lustre;
• Excellent: reflections are bright, sharp and distinct
• Good: reflections are bright but not sharp, and they are slightly hazy around the edges
• Fair: reflections are weak, hazy and or blurred
• Poor: reflections are dim and diffused.
The highest quality pearls have a sharp, mirror-like luster showing a high level of reflection.