According to modern scientific investigation, the formation of the pearl does not seem to be the result of healthy natural causes, but comes from the efforts of the oyster to rid itself of some foreign substance, like a grain of sand, a bit of shell or vegetation, or some unwelcome visitor in the shape of a small water insect.
When annoyed by an intruding substance, the oyster begins to deposit its nacre, or mother-of-pearl, in regular concentric layers around the intruder, these layers gradually increasing in circumference and forming the pearl. Thus, like an onion, the pearl is merely a succession of layers or skins, starting from a small core, or nucleus, which is always present, though often only of microscopical size.
Pearls have sometimes been found where the outer layer, or skin, as it is technically called, has been discolored or otherwise injured, and when this top skin has been carefully removed the result was a somewhat smaller but perfect pearl.
This, however, is a very delicate operation, and at the pearling grounds is only resorted to by men of experience. The composition of the pearl is carbonate of lime, with a small proportion of organic matter, and the specific gravity 2.5 to 2.7.
The pearl is affected by acids, and is easily calcined on exposure to heat.
In color, the pure white, slightly transparent, is the most highly prized; while in India and China the bright yellow colors are sought after.
Decided colors, however, such as black, pink, and golden-yellow bring a high price, and, in fact, black pearls, if perfect in color and shape, are at present more valuable than any other kind.
The beauty and value of a pearl depend on form, quality of texture or skin, color, transparency or water, and lustre or orient.
In form, the perfectly round shape comes first in value, then a finely formed drop or pear shape, and lastly the oval or egg shape.
Pearls that are flat on one side and rounded on the other are called boutons or button pearls. These are frequently found attached to the shell, and are cut out and the bottom part smoothed and polished.