and
the consecutive layers vary in brightness, colour and perfection. The
most highly prized Pearls are quite spherical, and it is evident from
their shape that these must have been formed free in the mantle or in
the soft tissues of the mollusc, and not cemented to the shell. Some
Pearls shew defects, caused apparently by the contact of new foreign
substances, organic or inorganic, such as grit or a film of weed ; and
in some cases it requires a number of layers to completely hide these
defects. Thus, every new layer secreted, changes the value of the Pearl.
When a Pearl that has been cut from the shell presents a hemispherical surface, it is sometimes called a perle bouton; such
a Pearl is flat on one side, and rounded or convex on the other. If a
solid Pearl has an irregular shape, having grown over a rough object,
it is known to jewellers as baroque pearl. Sometimes the warty Pearls are hollow, and pass under the name of coq de perle.
An
attempt has been already made in a former chapter to explain how the
Pearl-oyster rids itself of any foreign substance that may get into its
shell, such as a stone or piece of wood : and in now dealing with the
subject of real Pearls, the same explanation of the formation of "
blisters " must be borne in
