A pearl is said to be a hard object formed within the soft tissue of a living shelled mollusk. Similar to the shell of the mollusk a pearl is also formed of calcium carbonate in minute crystalline formed and is usually deposited in concentric layers. Pearls are generally round and smooth but can also come in various shapes. Only the finest natural pearls are said to be valuable gemstones.
Pearls are said to be the results of an adaptive immune system. When a foreign body gets inside the shell of a mollusk it usually irritates the soft tissues. So the mollusk creates a pearl sac around the foreign particle to seal off the irritation. It deposits layers of calcium carbonate around the irritation in form of concentric circles.
Pearls have an unique luster. It depends upon the ability of the pearl to reflect, refract and diffract light from the translucent layers. The luster also depends on the number of layers and how thin they are. The thinner they are, the finer the luster. The property of iridescence depends on the overlapping of successive layers which breaks up light falling on its surface. Artificially cultured freshwater pearls can be dyed yellow, green, blue, black etc. But the best natural pearls have a fine metallic luster to it.
Why black pearls are rare?
Black pearls are very expensive. Natural black pearls can be very rare. Rarer than the white pearls. Although it can be artificially produced but it will lack the dark, eerie and mysterious aura. Natural pearls are produced accidentally. When a foreign body, may be a sand or a parasite gets into the soft tissues of a mollusk it creates a disturbance. A sort of irritation. The mollusk tries to ease the discomfort by depositing layers of calcium carbonate around the irritation in concentric circles. Pearls have the same luminous, iridescent property as the insides of the mollusk.
Black pearls are formed in the same way as white pearls. But only a special type of mollusk can create a black pearl. That is why black pearls are very rare. The Tahitian black lipped Pinctada margaritifera are the only ones to create black pearls. The nacre which is the interior of a mollusk is generally white or silver. But the Tahitian mollusk has a thick band of black in its nacre. If the pearl is formed near this band it will suck up the colour too. Thus forming Black Pearls!