Within the jewellery world, a lustrous pearl holds a special place. It is an organic gem that comes out beautiful and fully finished, requiring neither cutting and faceting nor polishing. But what truly sets it apart from all the other gems is its lustre, the glossy and shiny quality that makes pearls seem almost magical.
Among all pearl value factors, lustre always comes at the top. Simply put, a highly lustrous pearl will always be more valuable than a similar pearl with poor lustre.
Lustre gives pearls exceptional visual and aesthetic appeal. Bright and radiant pearls are more attractive than dull ones.
High quality pearl lustre is usually described as sharp, bright, metallic, “ball-bearing like,” mirror-like, highly reflective and even glassy. By comparison, poor quality pearl lustre often appears dull, lifeless and chalky.
What is Pearl Lustre?
Pearl lustre is the bright, gleaming radiance that envelops a nacreous pearl. The allure of pearls is embodied in this characteristic.
Lustre results from the way light is reflected off of the surface, and just under the surface, of a pearl. It is also seen in the sharpness of the reflections that are visible on the surface of a pearl.
Why is lustre important? Because is the first thing that we notice when we look at a pearl. And, unlike other gems which need to be studied through the loupe or microscope to determine their quality characteristics and value, a pearl’s lustre can easily be detected by the naked eye. If a pearl appears bright and glossy, it is considered lustrous.
Pearl Lustre Grading
One of the easiest ways to assess the quality of a pearl’s lustre is to observe the reflections on its surface. Clear and sharply detailed reflections are the result of excellent lustre. But pearls with poor lustre show hazy and dim reflections.
The Gemological Association of America (GIA) grades pearl lustre in its reports on the following continuum:
Excellent – Pearls with excellent lustre exhibit bright, clear and sharp reflections on their surface. Such pearls are usually top quality pearls.
Very Good – Pearls with very good lustre show bright reflections that are near sharp but not completely crisp.
Good – Pearls with good lustre have bright reflection but these are not sharp and have same blurring or hazing around the edges of the reflected objects.
Fair – Pearls with fair lustre have weak and blurred reflections.
Poor – Pearls with poor lustre have dim, hazy and diffuse reflections.
Based on this continuum, it is easy to assess and grade the lustre of pearls.
Lustre and Pearl Prices
Pearl lustre is the most important value factor for pearls. As a result, it is also an important determinant of the price of pearls. Pearl lustre and pearl prices work together. High quality lustre leads to higher prices.
For instance, a strand of South Sea pearls with excellent lustre will sell for a much higher price as compared to another strand with similar (in size and colour) but fair or poor lustre pearls.
Factors Affecting a Pearl’s Lustre
Lustre is a combination of the reflection and refraction of light when it meets a pearl’s surface. The thickness of a pearl’s nacre and its surface quality play crucial roles in this process.
Thickness of Nacre
When we describe lustre as the shine or glow of a pearl, we’re actually referring to refraction1—how effectively light bends as it passes through the pearl’s concentric layers of nacre just below the surface, before reflecting back to the viewer. This makes it seem as if the pearl is glowing from within. Pearls with thicker and more tightly packed nacre layers refract light more efficiently, resulting in a more pronounced lustre.
Light refraction also gives pearls their unique iridescence and secondary colours. The nacre layers work like tiny prisms, breaking up light as it is reflected back to the surface of the pearl. This creates the subtle, colourful tones that shimmer across the surface, resulting in shimmering iridescence and superb secondary or overtone colours.
For some pearls with thick and tightly packed nacre layers, effective light refraction can even create a sublime orient. This is a shimmering, soap-bubble like optical effect, that appears as a rainbow of colours that skim on the surface of the pearl.
Therefore, pearls with thin nacre often appear chalky and lifeless, while those with thick nacre exhibit a rich glow and iridescence, alluring secondary or “overtone” colours and even a subtle orient.
Pearl Surface Quality
The surface quality also affects the quality of a pearl’s lustre. Pearls with blemishes, pits and other surface imperfections will not have a smooth surface. This will hinder light from effectively bouncing or reflecting off of the surface of the pearl. As a result, pearls with poor surface quality will not be very lustrous while pearls with smooth and shiny surfaces will often be very lustrous.
In addition, a pearl’s round shape acts like a convex mirror, reflecting light in a way that makes it appear to glow from within. Hence, highly lustrous pearls are sometimes described as having “mirror-like” lustre.
High Lustre Baroque and Keshi Pearls
We often associate high lustre with perfectly round pearls. However, some baroque and many keshi pearls are also highly lustrous even though they are spherically symmetrical. This is once again because of refraction.
Because of their irregular and asymmetric shapes, light is able to hit the surface of baroque and keshi pearls at many different angles. Light then refracts off off the different planes in many, separate directions, resulting in a greater amount of light being reflected back. Consequently, these pearls often appear particularly glossy and gleaming.
Different Types of Pearls and Their Lustre
Different types of cultured pearls have their own distinctive pearl lustre qualities.
Akoya Pearls
Akoya pearls are the most lustrous among all cultured pearls. These saltwater pearls are cultivated in the cold, temperate waters and harvested in winter. The lower temperatures slow down the metabolism of the oyster. This leads to the creation of more tightly packed and compact nacre layers.
This particular type of nacre deposition gives Akoya pearls their sharp, bright and mirror-like lustre, because these compact nacre layers reflect and refract light more effectively.
Within Akoya pearls, there is a special category which is called Hanadama Akoya pearls. These pearls are famed for their exceptional lustre which has a high-gleam and almost metallic quality to it. As a result, the lustre of Hanadama Akoya pearls is often compared to the shine of ball-bearings.
Freshwater Pearls
Tissue-nucleated Freshwater pearls comprise of layer after layer of nacre. Top quality Freshwater pearls have a soft lustre, which is similar to the sheen of high quality silk.
By comparison, bead-nucleated Freshwater (Edison) pearls have a sharper and more mirror-like lustre. Some bead-nucleated Freshwater pearls have an almost metallic lustre which gives them highly reflective surfaces and an intense gleam.
Tahitian Pearls
Tahitian pearls have a very distinctive, satiny lustre which has a luxurious feel to it. This is because Tahitian pearls have a thick nacre, but because of the warmer water temperatures where these pearls are cultivated, the nacre layers are not very tight and compact. Hence, when light interacts with the surface of these pearls, the lustre produced is deeper, but with a satiny and sumptuous quality.
Even so, gem quality Tahitian pearls can exhibit very sharp and crisp lustre, which is almost metallic in quality.
South Sea Pearls
South Sea pearls, similar to Tahitian pearls, have a softer, deeper and more lush lustre. These pearls also have a thicker nacre and are farmed in the warmer waters of the South Pacific. As a result, their nacre layers are less tightly packed, giving them a softer, satiny and more opulent lustre.
Why Are We Attracted to a Pearl’s Lustre?
According to Evolutionary Aesthetics, a school of thought that links human evolution to aesthetic perception, experience and design, the visual appeal of shiny and glossy things comes from the innate human need for fresh water as a resource for survival.
As a result, humans are naturally more attracted to glossy objects as compared to ones that are dull. Hence, the first thing we notice about pearls is their lustre or shine and the shinier the pearl, the more attractive it appears to us.
Can Pearls Lose their Lustre?
Yes, pearls can lose their lustre if proper care is not taken when wearing, cleaning and storing them.
Pearls are composed of calcium carbonate, which is alkaline in nature. As a result, exposure to acids or harsh chemicals will reduce their lustre. In addition, chips, scratches and cracks on a pearl surface will also reduce its lustre.
Here are some tips to keep your pearls shiny, bright and lustrous:
- After wearing pearls, gently wipe them with a slightly damp, soft cotton cloth to remove skin oils and sweat that can dull their shine. This will keep them clean and gleaming.
- Do not use any strong and/or abrasive chemicals to clean your pearls. For instance, do not use ordinary jewellery cleaning liquid, soap and glass cleaners to clean your pearls. The chemicals in these products will erode the surface of your pearls, reducing their lustre.
- Do not clean your pearls in ultrasonic cleaners. The heat and vibrations of these cleaners can cause the pearl nacre to crack and chip, damaging the pearls and making them lose their lustre.
- Always remember to put on your pearls after putting on perfume and hair spray.
- Pearls are made up of 2% – 4% water and dry out if they lose this moisture. Dried out pearls lose all their lustre, and some may even crack and break. Avoid keeping your pearls in airtight plastic boxes and lockers to keep them lustrous and beautiful.
The best ways to keep your pearls lustrous and beautiful is to wear them often.
A Pearl and Its Lustre
Pearl lustre is nothing more than the result of the interaction of light with a pearl. Yet, it is what makes pearls beautiful and unique. Factors such as a pearl’s nacre thickness and its surface quality affect the quality of its lustre. Therefore, while lustre adds to the visual appeal of a pearl, it is also a sign of its high quality.
Featured Image: Lustrous Baroque White South Sea Pearl, The Bows Collection, Mikimoto
- Light refraction also gives pearls their unique iridescence and secondary colours. The nacre layers work like tiny prisms, breaking up light as it is reflected back to the surface of the pearl. This creates the subtle, colourful tones that shimmer across the surface, resulting in shimmering iridescence and superb secondary or overtone colours.
For some pearls with thick and tightly packed nacre layers, effective light refraction can even create a sublime orient. This is a shimmering, soap-bubble like optical effect, that appears as a rainbow of colours that skim on the surface of the pearl.refraction ↩︎
