Four pearls, and one approximately
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The guides define four kinds of pearl and the first is the long one
A calcareous concretion consisting essentially of alternating concentric layers of carbonate of lime and organic material formed within the body of certain mollusks.
Then a cause, and the cause is described as abnormal
The result of an abnormal secretory process caused by an irritation of the mantle of the mollusk following the intrusion of some foreign body inside the shell, or due to some abnormal physiological condition in the mollusk.
Which is a definition by pathology
A pearl is defined as the product of something going wrong. No other definition in this library describes its subject that way, and it is accurate rather than colorful.
And then the clause that does all the work
Neither of which has in any way been caused or induced by humans. A pearl is a pearl because no person was involved.
So the identical physical object can be two different things
A cultured pearl is the composite product created when a nucleus planted by humans inside the shell or in the mantle is coated with nacre by the mollusk. The mollusk does the same work either way.
Which makes the distinction a matter of provenance and not of substance
This library has seen that before. Virgin wool is defined by never having been reclaimed, which is also a history rather than a property. Both are claims nothing about the object settles.
An imitation pearl is defined by appearance alone
A manufactured product composed of any material or materials that simulate in appearance a pearl or cultured pearl. Any material at all, and the only criterion is how it looks.
And the fourth definition is the one with a number in it
A seed pearl is a small pearl, as defined in paragraph (a), that measures approximately two millimeters or less.
Approximately is the word that makes it unique in this library
Every other measured threshold described across these properties is exact. 0.009 percent. 3.5 seconds. 19 microns. 500 milliliters. 118 milliliters. Fifteen percent of surface area. This one is approximately two millimeters.
Which is a deliberate choice given what it is measuring
A pearl is not round and not uniform, so a figure to the tenth of a millimeter would be spurious. The definition says approximately because the object does not support better, and saying so is more honest than a false precision.
And the companion section protects the facts behind a cultured pearl
It is unfair or deceptive to misrepresent the manner in which cultured pearls are produced, the size of the nucleus artificially inserted, the length of time the product remained in the mollusk, or the thickness of the nacre coating.