Pearls are undoubtedly the most costly and important of “organic” gems. They have been known since time immoral in the Orient and were known to the Greeks and Romans, evidently following the conquests of Alexander the great. This is type formed by accident, without human intervention, and was virtually only one known before the beginning of the twentieth century.



Appearance:
Most natural pearls used in jewellery are roughly spherical; this is the most suitable shape for ordinary necklace. Pearls may, however, be somewhat irregular in shape. If they have rounded not to obvious projections, they are known as baroque pearls. When examined under a10x or 20x lens, pearls often displays small, superficial irregularities, roughly conscious protuberances, barely visible furrows arranged in “parallel” or tiny flaws like miniature creatures on the moon, sometimes with the comet like tail on one side. At higher magnification the normally smooth and shiny surface displays closest, minute, sinuous lines, evenly distributed throughout. The color varies from white with a hint of grey to white with a yellow tinge, but can also be silvery grey or more noticeably yellow in strong light, pearls have characteristic “pearly” or “nacreous” luster and may also be iridescent, with the emphasis on pink or other colors, which give a very pleasing effect. Many antique pearls look badly damaged. Sometimes part of outer surface of nacre has been gone away or their may be loss of luster, caused by dehydration of the organic component or by the dulling affect of acid precipitation on the mineral component.



Distinctive features:
Pearls can generally be distinguished by their surface appearance-which is lustrous but with microscopic, discontinuous wavy lines-from glass imitations with the very different surface or from other imitations covered by a special, minutely granular varnish made from ground fish scales. A much harder problem is pierced natural from cultural pearl. Individual pierced pearls can be distinguished by observing the inside of the hole with a strong lens of binocular microscope. A succession of concentric layers(possibly with a dark center, if the pearls is slightly grayish) is characteristics of natural pearls, while a compact, almost waxy-looking nucleus, with a single, clearly different layer around it is characteristic of culture pearls. Although it is impossible to judge a single and un pierced by its outward appearance, in the case of a string of, say, a few dozen to a hundred or so pearls, simple observation of their outward appearance can be conclusive. Infact, although natural pearls are almost spherical, if one looks closely at a string of natural pearls, they nearly always appear to be bodies revolving about an axis (along which the whole is drilled) slightly flattened at one or both poles, perhaps even tending to a cylindrical shape. On the other hand, most although not all cultured pearls are more or less spherical, even when they have superficial irregularities and protuberances. This is because they consist of few layers of nacre on a strictly spherical support. Non spherical cultured pearls which are very familiar in shapes to most natural pearls, do occur but are not common and, as a rule, there will be very few on a string, the physical properties of pearls are not easily measured (except for the density) and are not normally used for recognition; but, on average, natural pearls have a slightly lower density than cultured pearls. Grayish natural pearls have a still lower density due to an excess of conch Olin. To distinguish natural from cultured pearls with any certainty, specialist laboratories use both radiography and the x-ray diffraction method, which give precise information on the arrangement of the internal layers of nacre and the prismatic crystals of aragonite of which are they composed.



Occurrence:
Most of the few natural pearls harvested nowadays come from the Persian gulf, Sri Lanka, the red sea, and the Philippines; still smaller quantities are collected from the sea of the coast of Venezuela and from the gulf of California. On other places such as the seas of Japan and along the north west coast of Australia, the industry for cultured pearls have now develop to such an extent that the possibility of finding natural pearls as well is disregarded.



Value:
One of the most valuable gems in antiquity, pearls is still highly valued today, although not to the same extent. They are evaluated according to size, color, luster, regularity of form, compactness (the more watery, translucent ones are less durable, therefore less valuable). In the case of the number of pearls in a piece of jewellery much depend on their uniformity of color or, at any rate, how well matched they are. A string of pearls of equal diameter is worth much more than one consisting of pearls of graduated diameters (larger at the center, smaller at the end), because numerous pearls of are uniform size are harder to find. Even a pair of matching pearls is worth more than double the price of a single pearl because of the quantities that have to be sorted to find two that are identical. But many natural pearls are old or antique and when they are in a poor state of repair, dehydrated or crack, brittle or yellowed with age, their value is greatly reduced.



Simulants:
Cultured pearls are not really imitations, but something much better. We shall, therefore, discuss these fully below. Pearls have been imitated, atleast since the mid-seventeenth century; hollow spheres of thin glass coated on the inside with the special varnish made from fish scales, and usually filled with paraffin or wax, were used. For this reason pearls were once tested between the teeth: if they broke, they were clearly false. They were subsequently imitated by (solid or hollow) spheres of glass, or mother/of/pearl, varnished on the outside in the same way. All these imitations are easily recognizable very easily if observed under a magnifying glass, which shows the typical features of glass fusion around the hole, the paraffin wax filling, and the translucent layer in one case, and, in the other, the minutely granular appearance of the special varnish used. From a distance, however, they look very much like real pearl, and there is a thriving industry for them in both in Japan and Majorca.