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Grading and valuing pearls

Part of the beauty of pearls is the natural process by which they are created. Like anything that is naturally grown, every single pearl is unique, with subtly different qualities, giving each pearl, or string of pearls, its own distinct value.

The Gemological Institute of America (GIA) grades pearl quality according to several different qualities:

  • Size
  • Shape
  • Lustre
  • Colour
  • Surface
  • Nacre
  • Matching

However, these different qualities carry different weights when it comes to value. For example, a highly lustrous yet smaller pearl will generally be worth more than a dull but larger one. To help you to understand how pearls are graded and valued, lets take a look at each of the GIA aspects in more detail:

Pearl Size

With pearls, size really does matter. Larger pearls take longer to grow, involving more investment of time and resources, and involving greater risks from disease and predators, so naturally there are less of them around and they are more valuable.

Size is measured in mm across the diameter of the pearl, at right angles to the drill hole. Strings of pearls are often given a size range, such as 6 – 6.5mm, indicating the range of minor variations between individual pearls.

Freshwater pearls average 6-7mm, and are rarely larger than 10-12mm. Small differences at average size don’t make much difference to the price, but above 7mm a single millemetre can double the price of a pearl.

Tahitian and South Sea pearls are usually much larger, with sizes ranging from 8mm to 20mm or more and are measured in whole millimetres. Once again, the larger the pearl, the greater the value, with prices rising significantly towards the largest sizes.

Recent developments have led to much larger freshwater pearls, cultured with a bead nucleus of 12mm or more, which can compete with large Tahitian and South Sea pearls.


Since pearls are natural gems, they are rarely perfectly round, even when they have been seeded with a round bead nucleus. The closer the pearl is to round, the higher the value, but there is also a strong market for other pearl shapes, such as drop, oval and even random shapes.

Pearl shapes can be spherical, symmetrical or baroque, which is the term for random shaped or free form pearls. The longer a pearl is left to grow, the greater the chance of it becoming misshapen through natural processes, acquiring a tail or otherwise deviating from the perfectly round ideal. Most wild pearls are off-round, semi-baroque or baroque in shape as they grow from randomly shaped nuclei, such as debris or parasites.


Perhaps the most important, and beautiful aspect of a pearl is its lustre, the sheen that makes them shine, and this quality is valued above all others.

A pearl’s lustre comes from the way light plays off the many layers of translucent nacre within the pearl. The deeper the nacre, the more the light will reflect and refract, and so the higher the lustre of the pearl. The lustre of a pearl not only reflects light, but also reflects the level of care and attention given to the mollusc by the pearl farmer during the pearl’s growth.

The lustre of a pearl creates the reflections and produces the rainbow of colours, known as iridescence, that dance across its surface like oil on water. Low lustre pearls will look milky and dull, while high lustre pearls will shine and dance before your eyes.

All pearls from Absolute Pearls are selected for very good to excellent lustre as we believe that this is one of the most important qualities of a pearl.


Think of a pearl and you probably think off-white, but that’s just half the story. Many pearls are indeed white, but they can also be yellow or gold, pink or lilac, silver or even black. It all depends on the mollusc they grow in.

Even white pearls will vary hugely in their subtle shades, known as overtones. Like the popular ‘white with a hint of…’ paint shades we love on our walls, white pearls can have subtle overtones in a whole spectrum of colours.

Generally speaking, the whiter the pearl the higher the value, but there are many exceptions to this rule, such as Tahitian black pearls and the golden pearls of Indonesia. The rarest Tahitian pearls, in an oily peacock green, are highly sought after.

Pearl colour is also a matter of taste, as we discuss elsewhere in our discover section, with different colours matching different skin tones.


As a natural gem, most pearls will have some minor flaws and blemishes, and perfect or flawless pearls are extremely rare. In many ways, a few minor blemishes are welcome as a reminder that your pearl is natural and unique.

The value of a pearl depends on how extensive and how noticeable these natural blemishes are, with the cleanest pearls worth the most.

Anything that interferes with the growth of the pearl, such as a parasite or the internal structures of the mollusc, will cause a blemish on the surface as the pearl grows around it. Blemishes range from small pits and bumps, to larger ridges and creases and even full rings around the pearl. Significant obstructions will cause the pearl to deform from its round shape as it follows the path of least resistance, creating a baroque pearl.

All pearls from Absolute Pearls are selected to be either virtually blemish free or with minor blemishes. We pay a premium for this but we believe it is worth it to give you the best quality that we can.


Nacre is the substance secreted by the mollusc, which gives pearls their unique appearance, and the deeper the nacre, the higher the lustre of the pearl.

Freshwater pearls are 100% nacre, however the depth of seeded saltwater pearls can vary considerably, depending on how long they have been allowed to grow. If a pearl is harvested too soon, the nacre will be thin and of a poor quality, affecting not only the appearance, but also the longevity of the pearl, leading to cracking and flaking.

You should aim for 0.3mm of nacre on a saltwater pearl as an absolute minimum, although 0.5mm or more is preferable. Some Tahitian pearls have a nacre depth of 0.8mm or more, creating spectacular lustre, iridescence and overtones.

Pearl grading systems

There is no standard system for grading pearls, with different dealers using different, and highly subjective, grading systems. For details of how we grade our pearls at Absolute Pearls, please visit our buying advice section.