ORIGIN OF PEARLS

The origin of pearls was a subject of much speculation by many early writers. The belief that they were formed by drops of rain falling into certain shells and afterwards hardened by a secretion of the oyster was quite common. The real composition of a pearl and the method of its natural formation are, in fact, something which only comparatively recent scientific research has revealed to us. We now know that this beautiful gem is the product of "a mollusc, a shell fish, and although of organic origin, it is com­posed principally of mineral matter. For this reason, it cannot be strictly regarded as a gem stone, although it must be recognized as a precious gem.

All bivalves, that is, those shell fish which possess two half shells, are capable of producing pearls, but as so few pearls are actually found, it is necessary to say something about the animal which first builds the shell and later, under certain favorable con­ditions, also produces the pearl.

The origin of pearls was a subject of much speculation by many early writers. The belief that they were formed by drops of rain falling into certain shells and afterwards hardened by a secretion of the oyster was quite common. The real composition of a pearl and the method of its natural formation are, in fact, something which only comparatively recent scientific research has revealed to us. We now know that this beautiful gem is the product of "a mollusc, a shell fish, and although of organic origin, it is com­posed principally of mineral matter. For this reason, it cannot be strictly regarded as a gem stone, although it must be recognized as a precious gem.

All bivalves, that is, those shell fish which possess two half shells, are capable of producing pearls, but as so few pearls are actually found, it is necessary to say something about the animal which first builds the shell and later, under certain favorable con­ditions, also produces the pearl.

The pearl oyster belongs to the mussel family, the most impor­tant varieties being the margaritifera vulgaris, which yields the fine Persian, Red Sea, and Malay pearls, the margaritifera max­ima, which produces the Australian pearl, and the margaritana margaritifera, which is found in rivers and which therefore gives us the so called fresh water pearl. The mollusc known as the ormer and the American abalone are both fished for their shells. 

The pearl oyster resembles the scallop in shape, and during the bi-yearly spawning seasons, millions of these young oysters are liberated. Only some survive; these may fall on rocks or other submarine objects, to which they are able to cling firmly by means of their byssus, a glandular secretion which takes the form of a bundle of tough threads or fibers. 

If conditions continue to remain favorable, the young oyster begins gradually to build its home—the outside shell. The two flaps of soft tissue which cover the animal are covered with minute cells. This "mantle" pours out a mixture of horny materials and carbonate of lime, and four separate layers build up the shell. The outer layer, known as the periostracum, consists of a horny mate­rial, conchyolin. This is produced by the cells of the mantle. The second layer is the prismatic layer, and this is deposited on the first layer. It consists of calcite or aragonite with conchyolin, ar­ranged in the form of prisms at right angles to the surface of the shell. The third layer, the nacreous layer, consists of calcium car­bonate (mother of pearl), which is secreted by the entire external surface of the mantle, and this is built up of delicate lamellae which overlap and lie parallel to the surface of the shell. The edges are irregular and exposed, and this causes light interference. thus producing the beautiful iridescence so admired in pearls. The fourth layer, the hypostracum, is found between the muscle of the fish and the shell. It is composed of columns of calcium carbonate built up at right angles to the surface of the shell. 

These layers take some time to grow and form, and when the shell is completed, the oyster would do little else but remain in its home and look for food. The shell is formed of two pieces of approximately the same size and shape, these being hinged at one end. A powerful adductor muscle keeps them closed, but when food is required, the muscle is relaxed and the shell opens. Any suitable food that passes by is considered by the oyster, but if, by accident, a foreign body such as a sand grain, a parasite, or a frag­ment of rock gets into the valves, then an irritation is set up, and the oyster tries to expel or isolate the irritant from its soft body. It may be ejected, but if this is impossible, the oyster begins to cover it with layers of shell material of exactly the same compo­sition as its outer shell covering. As a result of such an accident, the gem we know as the pearl is created. 

It will be realized that the nucleus of a natural pearl, generally a fragment of horny material, is minute, and that a considerable time must elapse before the concentric layers grow into a pearl of any size. Other conditions, including the absence of enemy fish, must also be present. True pearls, or cyst pearls, are found embedded in the mantle or soft tissue of the oyster; they are un­attached and therefore spherical or pear shaped. These are the most valuable pearls. Irregular shapes, known as baroque, are of little value. Drop and button shaped pearls are not so valuable as round pearls since they cannot be used in necklaces; their chief use is in earrings, pendants, and ring's.

If the irritant gets between the shell and the mantle, or if a boring parasite pierces the shell, the pearl produced has to be cut: away from the shell. Such pearls, known as blister pearls, are often irregular in shape and flattish; they are of little value, and find a use in the cheapest forms of pearl jewelry. 

Size, shape, color, and luster determine the value of a pearl. Perfectly round specimens are the most valuable, and the bright, warm rosee tint is the most highly esteemed. In general, it may :be said that the whitish pearls are in less demand, as are also the definitely yellowish. A true black pearl, but not the gray, is valuable; "fancy" tints such as brown, bluish, and greenish are not highly appreciated. Colors vary considerably from white to black; Luster varies with the nature of the layers, and different ocean beds 

seem to produce pearls of characteristic tints. An expert can tell from which area a pearl originated solely by its sheen, just as the diamond expert can generally determine the location in which a diamond was mined.