Pearls have a rich inner glow that seems to come from deep within. That’s luster. It’s an effect caused by light reflecting and diffusing through the layers of nacre. High luster tends to make a pearl more valuable. As a test, stand with a beam of bright light behind you, hold a large, fine-quality cultured pearl close to your face. You’ll see the light reflected and yourself.
The play of light refracted through many layers of concentric nacre creates the orient – the iridescence of a pearl that sometimes creates a rainbow effect. The Latin word oriens means “the rising of the sun”.
A pearl and the inside surface of the shell of its parent oyster or mussel are made of nacre. Layer by layer, this crystalline substance builds up around a small bead or piece of shell (the nucleus) implanted in the culturing process. The thicker the nacre, the more lustrous and durable the pearl is. Through a microscope, the surface nacre of a cultured pearl looks like a lunar landscape – irregular “fingerprints” that are unique.
To check if a new pearl is “real”, rub it gently against your front teeth. If it feels bumpy or gritty, it’s probably a genuine cultured pearl. A simulated or imitation pearl will glide over the teeth and feel like plastic.
Most pearls are measured by their diameter, irregularly shaped pearls by their width and sometimes length. The standard of measurement is the millimeter (mm). From less than 1mm (seed pearls) to over 20mm (South Seas & sometimes Tahitians), the size of a cultured pearl depends mainly on the size of the parent oyster or mussel, the size of the implant nucleus, and how long it is cultivated. All factors being equal, the larger the pearl, the greater the value. For instance, a Mabe cultured pearl has a wide diameter but is a “half-pearl” with a flat bottom. Its value is much less than a large, round South Sea cultured pearl of similar quality.
Some average sizes:
- Akoya 2-10mm
- Tahitian 8-16mm
- South Sea (White & Golden) 8-18mm
- Keshi 1-10mm
- Mabe 12-25mm
(One millimeter = 1/25 inch)
Cultured pearls come in an almost infinite variety of shapes, partly determined by the shape of the nucleus around which the pearl is formed. Round pearls are the traditional favorites. A pearl with an irregular shape is called Baroque. The popular pear and teardrop pearls are Semi-Baroque - irregular but symmetrical. No matter what the shape, it falls into one of five main categories.
Symmetrical spheres. Variation rate of diameter – 2% or less.
Almost spherical. Variation rate of diameter – 2 to 5%.
(Drop Button Oval Pear) Partially symmetrical. One axis of revolution or more.
(Primarily Tahitian & South Sea pearls) Streaks, ridges or rings in regular formation on the upper third (minimum) of surface.
To test the roundness of a pearl, place it on a smooth, sloped surface. The rounder the pearl, the straighter and more easily it rolls.
From opaline white to anthracite black, the palest pastels to vibrant hues, pearls come in almost every color and shade. The finest have a subtle interplay of colors – the primary body color plus delicate overtones. One of the rarest colors today is the rich warm gold of South Sea pearls from the Philippines.
What’s called “cleanliness” affects the value of a pearl. Generally, the fewer blemishes the better. Tiny irregularities are acceptable and can be a test of authenticity.
The value of a pearl is seldom judged by weight. Single pearls are weighed by grains or carats. One grain = 50 milligrams. Four grains = one carat. In the wholesale market, strands of cultured pearls are often sold by the "momme" (pronounced moh-may).
Momme is a weight measure developed by the Japanese centuries ago. One momme = 3.75 grams. Today, momme weight is still the standard unit of measure used to communicate with the Asian pearl suppliers and pearl farmers. Momme is pronounced much like the word "mummy." One momme = 1/1000 kan. Reluctant to give up tradition, in 1891, the Japanese government formalized the kan measure as being exactly 1 kan = 3.75 kilograms or 8.28 pounds. Hence, 1 momme = 3.75 grams or 3750 milligrams.
= 4 grains = 200 milligrams = 1/5 gram
= 1/4 carat = 50 milligrams = 1/20 gram
= 18.75 carat = 3750 milligrams = 3.75 grams
= 18,750 carat = 3750 grams = 3.75 kilos
PEARL COHERENCE - MATCHING
Each cultured pearl is unique. To create jewelry, pearls that complement or match each other are selected in a process that can take years. Thousands of pearls may be rejected to find the desired combination. When the results are successful, the piece has balance and works as a harmonious whole. It has coherence.
Japanese Akoya Pearls Luster
Nacre and luster are the most important quality measures. Nacre quality is determined by analyzing layer thickness and layer density. Both of these factors help us to determine luster. Since the average consumer is not able to determine nacre quality, the most import factor for the consumer to understand is luster, which can be judged easily by the naked eye.
Pearls with high luster sharply reflect images around them, almost like a mirror. They always display the characteristic depth, or three-dimensional glow, and a subtle display of different surface colors (orient) that is so prized among high-quality pearls. Pearls with low luster often look dull, chalky and lifeless.
Luster, in more scientific terms, is the reflection and refraction of light as it passes through the layers of aragonite platelet crystals that form a cultured pearl’s nacre. The intensity of luster is determined not only by the amount of these crystals but also by the specific geometric pattern and regularity in which they are secreted on the nucleus by the mollusk.
Many factors affect nacre secretion, determining the intensity of luster. The health of the oyster, the water temperature of the host environment and the level of nutrients available to the oyster all play a vital role in creating a pearl with superior luster.
Though the intensity of a pearl's luster can be an indicator of nacre thickness, and thus determine the durability of a pearl, this correlation is not always true. Some cultured pearls have very thick nacre combined with poor density resulting in mediocre luster. This goes back to the formation patterns of aragonite platelets. Platelets in a very loose formation cannot refract and reflect light as well as very tightly packed platelets (nacre density). The loose formation phenomenon occurs more often in pearls grown in warm water.
Chinese Akoya pearls are cultivated in warm waters throughout the entire nacre growing period. This promotes quicker/more nacre coating, but the coating is often what many experts call “soft”, non-durable and typically lacking good luster. The waters in which the Japanese oysters are farmed are much less polluted and subject to broader seasonal changes in water temperature. Cold water slows down the nacre secretion process, reducing the size of aragonite platelets and producing tighter, more densely packed nacre layers. Japanese producers conduct their harvest only once a year in the freezing winter weather to ensure that the cold-water pearl coating is at its peak. Temperature combined with minimal pollution gives the fine quality Japanese Akoya pearls their “mirror like” luster.
Images by Mastoloni Pearls.