The following explanations will allow you  to understand the different factors influencing the value of pearls. It is important to know that there is no official international scale of  pearl grading (like for diamonds  for example. So these classifications can vary from one producing country to another and from one gemologist to another). 

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CHANDELIER|SURFACE QUALITY|FORM|COLOR|SIZE|NACRE THICKNESS|LAYOUT|VARIETIES|TREATMENTS

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Chandelier

The luster of the pearl is undoubtedly the main quality criterion on which to dwell. It refers to its brilliance and the way the pearl has to reflect light. Luster is usually graded on the following scale: Exceptional (metallic), Excellent, Very Good, Good, Fair, and Poor.

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Surface quality

The surface of the pearl represents the outer layer of the nacre. The less imperfection there is on the surface of the pearl, the higher the value will be. Some imperfections are more acceptable than others, for example a rimmed pearl might still retain good value if the rimming is visually appealing. It is the same for certain wrinkled baroque pearls. On another note, imperfections such as scratches, cracks, severe discoloration, holes, growths or a heavily pitted surface could drastically lower a pearl's value.

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Form

Although the perfectly round pearl is the most expensive, certain other shapes can also have significant value. Such as a symmetrically perfect pear, a "fireball" or a baroque with an interesting and unusual shape. "Potato" shaped pearls (neither round nor baroque, i.e. potato-shaped), buttons and oval pearls often present one of the best quality-price ratios in relation to the aesthetic aspect of the Pearl. Baroque pearls are also a less expensive choice providing an interesting visual.

  

However, a pearl with a perfect and symmetrical sphere will always be more expensive for equal quality than another shape.

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Color

The color of a pearl is subdivided into two categories either; the base (body) color and the complementary color(s). The base color represents the predominant color of the pearl, therefore the main one. Complementary colors define the perceptible shades of color besides the main color. For example, a white pearl could show pinkish, silvery, purplish, creamy or even greenish or bluish reflections.

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Some colors will have a higher value due to their rarity or if the demand at the moment is greater than the supply on the market. This is particularly the case with pistachio-colored Tahitian pearls or natural purple-colored freshwater pearls.

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Size

Usually the larger the pearl, the more expensive the price. It depends on the variety and the availability of a size according to the varieties. There are certain exceptions, such as Tahitian pearls under 8 mm  (between 7 and 8 mm) which, being less abundant and rarer on the market, are a little more expensive than beads from 8 mm to 9 mm.

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Pearl thickness

When we talk about the thickness of the mother-of-pearl of a pearl, we are referring to the layer of mother-of-pearl produced by an oyster above the core. As freshwater pearls are usually cultured without a core, this criterion only applies to saltwater pearls (Akoya, Tahiti, South Sea). It is particularly important in the case of Akoya (Japanese saltwater cultured pearl) because if the layer of nacre covering the core is insufficient, the pearl will present a poor luster and could even flake or peel.

A mother-of-pearl of 0.5mm and more is considered reasonable for an Akoya, while Tahitian and South Sea pearls should have a mother-of-pearl of at least 1mm.

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Layout

A perfectly matched strand or pair of pearls in terms of size, shape, color, luster and surface quality will always be more valuable. Particularly in the case of pearls of particular colors or quality which are difficult to arrange. This criterion also applies to the row of multicolored  or very baroque pearls which must nevertheless present a relationship in shape, size, color or luster.

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Varieties

As a general rule, saltwater pearls are more expensive than freshwater pearls due to their method of production. In salt water, pearls are grown in oysters  where only one or two grafts are placed at a time. In fresh water , mussels are used; bigger and stronger molluscs that can support up to a maximum of 60 pearls at a time.

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Treatments

The pearls can undergo several treatments which will have little or no influence on the price. Slight bleaching of white pearls is very common and will not harm luster if done in moderation. But if the bleaching is too strong, it will result in a very unattractive dull white pearl. Light polishing in large churns with cork pieces is also common practice for all types of pearls after harvest. Some scratches or slight surface defects could also be erased by a little more aggressive polishing.

Among the treatments that have a greater impact on the visual appearance of the pearl, there are notably dyeing and irradiation. They both have the purpose of markedly altering the color of the pearl. Irradiation is mainly used on freshwater pearls since containing isolated manganese atoms, they are the only ones to react when exposed to Gamma rays. With this treatment, you can obtain gray pearls or dark pearls (peacock) with different bluish, purplish or green reflections. It is also possible, on a smaller scale, to carry out this treatment on Akoya pearls since their core is composed of freshwater mother-of-pearl and their outer layer of mother-of-pearl is thin enough to let the color of the core appear transparent. We can therefore obtain pale gray pearls.

A dyed or irradiated  pearl will not be worth less than what it was worth before being treated, and sometimes even a little more due to the work carried out. But watch out for dark freshwater pearls that pass themselves off as a far more expensive Tahitian pearl or a pearl that is dyed gold to imitate the higher value South Sea pearl.