Cultured pearls is the term used to refer to pearls that are formed as a result of a shell’s response to some form of tissue implant. This article will look at how this is done and how the end product can be differentiated from naturally formed pearls.
The insertion of objects, such as figures of a Buddha, between the shell and mantle of molluscs in order to obtain a covering of nacre has been practised by the Chinese since the 13th century. In 1896, after years of work, Kokichi Mikimoto of Japan patented the process of producing cultured blister pearls, but it was not until the 1920s that round cultured pearls appeared on the market.
The culturing process
Today the production of cultured pearls is a large, highly organised business. Oysters are reared on huge farms until they are three years old and ready for the culturing process. They are removed from the water and have one or more spherical mother-of-pearl beads implanted, as well as a small piece of mantle taken from a sacrificial oyster. It is this section of foreign mantle that stimulates the oyster to produce a protective pearl sac and then to coat the bead with nacre. Up to half of the recipient oysters will reject their implant or die. Those successfully accepting the process will go on to repeat the procedure two or three times with ever increasing sizes of bead being used.
Formation of the pearl
The living molluscs are then returned to the water, ideally for about three years, before being harvested at winter time, which apparently gives the best results. At this time the bead will have been coated, first in a shell-like material called conchin and then by approximately 0.2 to 1.0 mm of nacre, the pearlescent finish that we all associate with pearls. In reality, due to increased demand, growth time has been drastically reduced down to as little as six months, resulting in many more cultured pearls, but of reduced quality.
Distinguishing cultured pearls from natural pearls
If a pearl is cut in half then it is easy to determine how it was formed by examining the interior. Cultured pearls show the flat or striped cross section of the mother-of-pearl bead depending on the direction of the cut, whereas natural pearls will show the concentric ring structure indicating a build-up of numerous nacre layers. Of course it is not always possible to cut open a pearl and so other methods are used. If the pearl has been drilled in order for it to be strung onto a necklace, one can inspect the drill hole under 10x magnification in order to observe the layers of surface nacre, the darker conchin layer and then the mother-of-pearl nucleus. Natural pearls will again show a concentric layered structure. A pearl that has not been drilled may be held up to the light in a process known as candling. Here we are looking for the thickness of the nacre and the banded structure of the mother-of-pearl bead as the pearl is lit from behind by the strong, diffused light of the candle.
Sometimes it is necessary to send pearls to the laboratory in order to be 100% sure of their nature. The most commonly used test for pearl identification in gemmological labs is X-ray testing and a variety of methods are used including X-radiography, X-ray diffraction and X-ray fluorescence. Pearls that have undergone such tests and been certified as natural can, depending on other factors, be worth vast sums of money. Auction prices for a necklace of certified natural pearls can reach £100,000 and far beyond.
Although cultured pearls are, to an extent, man-made, this cannot be said of another group of materials referred to as imitation pearls. These items may be beads of glass, plastic or shell, covered with a coating that imitates the appearance of nacre. This coating is known as guanine and is taken from a crystalline substance found in herring fish scales which is suspended in lacquer and painted onto the beads, usually in up to five to ten layers. 10x magnification can sometimes reveal chips in these layers or a build-up of the coating around the drill hole. The coating also appears granular in contrast to the overlapping platelets of a natural or cultured pearl. One other simple test is to run the pearl gently along the bottom of one’s front tooth - a natural or cultured pearl will feel gritty, whilst an imitation pearl will feel smooth.
If you have any pearls that you would like to sell, please contact Simon Rufus, gemmologist at Grand Auctions, Folkestone, Kent for further details.