ght to a certain
extent. The layers being exceedingly thin, light can penetrate a
considerable number of them if not otherwise deflected from its course.
We thus obtain reflections not merely from the outer surface of a pearl,
but from layer after layer within the gem and all these reflections
reach the eye in a blended reflection of great beauty. The luster of a
pearl is then not purely a _surface luster_ in the usual sense of that
term, but it is a luster due to many superposed surfaces. It is so
different from other types of luster that we describe it merely as
_pearly luster_ even though we find it in some other material, as, for
example in certain sapphires, in which it is due to a similar layer-like
arrangement of structure.

ORIENT. The fineness of the luster of a pearl, or as is said in the
trade, the _orient_, depends upon the number of layers that take part in
the reflection, and this number in turn depends upon the translucency of
the material and the thinness of the layers. Very fine pearls usually
have very many, very thin layers taking part in the reflection. The
degree of translucency, considered apart, is sometimes called the
"water" of the pearl.

In addition to their beautiful luster, many pearls display iridescence,
and this is due in part, as in the case of the pearly lining of the
shell (mother of pearl) to overlapping of successive layers, like the
overlapping of shingles on a roof. This gives rise to a lined surface,
much like the diffraction grating of the physicist, which is made by
ruling a glass plate with thousands of parallel lines to the inch. Such
a grating produces wonderful spectra, in which the rainbow colors are
widely separated and very vivid. The principal on which this separation
of light depends is known as diffraction and cannot be explained here,
but a similar effect takes place when light falls on the naturally ruled
surface of a pearl and helps produce the play of colors known as
iridescence. The thin layers themselves also help to produce the
iridescence by interference of light much as in the case of the opal,
which has already been discussed.

COLOR. Having explained the cause of the orient and water of pearls, the
_color_ must next be considered. Pearls may be had of almost any color,
but the majority of fine pearls are white, or nearly so. The fine
Oriental pearls frequently have a creamy tint. Among fresh water pearls
the creamy tint is less often seen, but fine pink t