Determining Pearl Quality
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.
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, lustre, 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.
Keshi pearls are a shape rather than a type of pearl as is commonly believed. They are formed when the oyster rejects and spits out the implanted nucleus before the culturing process is complete, or the implanted mantle tissue fractures and forms separate pearl sacs without nuclei. These pearl sacs eventually produce pearls without a nucleus. Keshi pearls can form in either saltwater or freshwater mollusks.
They are generally small in size and because there was no nucleus to guide the ultimate shaping of the pearl, their shapes vary broadly. Keshi pearls come in a wide variety of colours and tend to have high lustre and even orient. This is due to their solid-nacre composition - the implanted nucleus of the pearl has been expelled by the mollusk resulting in 100% nacre. This gives it an especially lustrous and shimmering surface quality. Most keshi pearls, in fact, have a greater lustre than even the best-quality cultured pearls. The fact that keshi pearls are solid nacre does not, however, give them the classification of natural pearls. This is because keshi are a bi-product of the culturing process, not a natural occurrence.
Keshi pearls, especially South Sea keshis, were once quite the bargain yet beautiful and unique pieces. Today South Sea keshi pearls are much rarer. This is because South Sea pearl farmers are now x-raying oysters to determine whether or not the nucleus has been expelled. When a nucleus-free oyster is found they are re-nucleated before a keshi has time to form. This practice has made South Sea keshi pearls much harder to find than was once the case. Keshi means "poppy seed" in Japanese and these pearls are often also referred to as "poppy seed pearls."
Pearls are measured by their diameter in millimetres. Tiny seed pearls can be smaller than 1mm, while South Sea pearls, as large as 20mm, have been found. If all other quality factors are equal, the size of a pearl will determine its value. Only a 1 millimetre increase in pearl size will be 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 11mm.
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, for example, the pearl develops against the shell it will become more flattened on that side. Some pearls 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 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:
- Round: Round pearls are perfectly spherical - the shape most people think of when they think of a pearl. Due to their relative rarity and "classic" nature, they are highly desirable. Round pearls fall into the spherical category.
- Near-round: These pearls are not perfectly round. Instead, they are slightly flattened or elongated rather than being a perfect sphere. Nonetheless, they are so nearly perfect that they, too, are classified as spherical.
- Semi-round: These pearls are semi-round, similar to near round.
- Oval: These pearls are shaped like an oval - narrower at the ends than they are in the centre. Ovals are categorised as a symmetrical shape.
- Button: Button pearls are flattened to some degree, making them resemble a button or perhaps a disk rather than a perfect sphere. These pearls are often used in earrings where the flattened side can be attached to the setting. Buttons are also categorised as symmetrical.
- Drop: Drop shaped pearls are pear or teardrop-shaped. The drop can either be "long" or "short" depending on its proportions. These pearls make attractive earrings or pendants. This is also a symmetrical shape.
- Baroque: This is a pearl which is both non-symmetrical and irregular in shape. The baroque pearl can be purely abstract in its shape or it can resemble a cross, stick or some other shape. Baroque pearls fall into the baroque category.
- Semi-Baroque: These pearls are slightly irregular in their shape. For example, a pearl which might otherwise be considered an oval, button or drop pearl but which is not symmetrical in nature, would be considered semi-baroque. Semi-baroque pearls fall into the baroque category.
The shape of the pearl is one of several factors which goes into determining its quality and, therefore, its value. In general, round and near-round pearls are the most valuable due to their rarity. Symmetrical shapes are generally considered to be more desirable than baroque shapes. Baroques, however, can be extremely unique thus increasing their desirability more than might be expected based on their shape alone. A perfectly round pearl is very rare. The rounder the pearl, the more valuable it is. Baroque pearls are not symmetrical in shape and can be lustrous and, therefore, just as appealing as their round counterparts.
A pearl's 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 - 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 a pearl's main body-colour. These overtones tend to alter the body-colour somewhat as well as adding depth and glow. For example, a pearl may be white with rose overtones.
Some pearls have no overtones at all while some display an orient that shows 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. Lustre is more than a combination of surface brilliance and a deep glow. It is caused by light travelling 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 or not 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 lustre showing a high level of reflection.
Pearl Surface Quality
Pearls are organic so it is not surprising that they show surface qualities. The cleaner the surface of the pearl, the more valuable it is. There are four classifications:
- Clean - pearls can be blemish free or contain minute surface characteristics that are very difficult to see.
- Lightly Blemished - minor blemishes or surface irregularities.
- Moderately Blemished - noticeable surface characteristics.
- Heavily Blemished - obvious surface irregularities.
Pearl Nacre Quality
Nacre is the very essence of the pearl itself. It is the natural substance that the mollusk secretes to protect its sensitive flesh from irritants such as shell fragments or implanted beads. Nacre is made up of thousands of layers of thin overlapping crystal plates with irregular edges. This is the same beautiful iridescent material that lines the inner surface of the oyster shells. A pearl's appearance helps determine its quality which has a lot to do with whether the nacre is thick or thin, although thick nacre does not always indicate a highly lustrous pearl. Nacre quality can be classified in three ways:
- Acceptable - nucleus not noticeable, although slight blinking might be present. No chalky appearance.
- Nucleus Visible - the pearl shows evidence of its bead nucleus through the nacre.
- Chalky Appearance - the pearl has a dull chalky appearance.
The lustrous outer surface, or nacre, of natural and cultured pearls is made up of the same nacreous material. Its chemical composition is about 90% calcium carbonate; the rest is water and organic materials.
Pearl nacre is composed of microscopic crystals. The crystals are aligned perfectly so that light passing along the axis of one is reflected and refracted by the others to produce a rainbow of light and colour. The iridescence that we commonly associate with pearls is produced by the particular arrangement of the nacre layers. The size and shape of a cultured pearl is determined by the size, shape and position of the implanted nucleus. The size, shape and colour of a natural pearl is determined by the type of irritant, the water condition and food supply. The general health and wellbeing of the oyster will also determine how lustrous the nacre of the pearl will be when harvested.
Pearl matching is more important when considering a strand of pearls. Minor variations between pearls within a strand should be expected as no two pearls are identical. When considering pearl matching, uniformity of pearl factors such as size, shape, colour and lustre should be taken into account.