.

In this connection may be mentioned the widespread belief on the part of
the public that the concretions found in the common edible oyster can be
polished by a lapidary, as a rough precious stone can be improved by the
latter, and that a fine pearl will result. It is frequently necessary
for jewelers to whom such "pearls" are brought, to undeceive the person
bringing them and to tell him that only those molluscs that have a
beautiful pearly lining to their shells are capable of producing true
pearls and that the latter require no assistance from the lapidary.

SHAPE. To return to the topic of factors governing the value of pearls,
the _shape_ of the pearl makes a vast difference in the value. Perfectly
spherical pearls are most highly valued and closely following come those
of drop or pear shape, as this shape lends itself nicely to the making
of pendants. Oval or egg-shaped pearls are also good. After these come
the button shapes, in which one side is flattened. Pearls of irregular
shape are much less highly valued. The irregular-shaped pearls are
called _baroque_ pearls in the trade. The rivermen engaged in the fresh
water pearl fishery call them _slugs_. Some of the more regular of these
are called "nuggets." Others are termed "spikes" because of their
pointed shape, and still others are called "wing" pearls on account of
their resemblance to a bird's wing. Most of the baroques are too
irregular in shape to have any special name applying to their form.

WEIGHT. After orient, color, skin, and shape have been considered,
_size_ or _weight_ finally determines the value. Pearls are sold by an
arbitrary unit of weight known as the _pearl grain_. It is not equal to
the grain avoirdupois, but is one fourth of a diamond carat. As the new
metric carat is one fifth of a gram and as there are 15.43 avoirdupois
grains in a gram, it is seen at once that there are but 3.08 real grains
in a carat rather than four. Thus the _pearl grain_ is slightly lighter
than the avoirdupois grain.

Since large, fine pearls are exceedingly rare, the value mounts with
size much more rapidly than is the case with any other gem; in fact, the
value increases as the _square_ of the weight. For example, let us
consider two pearls, one of one grain weight, the other of two grains,
and both of the same grade as to quality. If the smaller is worth say $2
per grain, then the larger is worth 2 x 2 (the square of the weight)
times $2 (the _price per gr