932 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
aragonite. These layers being semitransparent, the light falling on 
the surface is partially reflected from the surface and partially trans- 
mitted into the stone, where it suffers reflection from the surface of 
lower layers (pl. 3, fig. 4). 
To produce this complicated structure artificially is practically 
impossible, unless one can describe as an artificial pearl that formed 
by the oyster in response to the deliberate introduction of irritant 
foreign matter by human agency. But in this case who shall decide 
where nature ends and human ingenuity begins? Perhaps the well- 
known Japanese pearl may be correctly described as artificial pearl, 
although the oyster has a great deal to do with it. 
Such pearls are formed by introducing a piece of mother-of-pearl 
in the shape of a hemisphere between the shell and mantle of the 
oyster and then leaving the oyster alone for a time to allow it to 
convert this into a pearl by the deposition of several layers of nacre. 
The mass is then removed from the shell and converted into the 
semblance of a true pearl by supplying a back of mother-of-pearl. 
Such pearls, however, never have the fine orient of those produced 
under normal conditions, and they can readily be detected by exam- 
ining the back, when the lusterless mother-of-pearl and the line of 
junction can be detected. 
Of course, wonderful imitations of pearl are made in various ways, 
which are difficult to distinguish from the natural article by casual 
examination. One method of preparation is as follows: Small hollow 
spheres are blown in opalescent glass, coated inside with a prepara- 
tion of fish scales, and then filled up solid with wax. Such imita- 
tions are identified by examination of the hole or by putting a spot 
of ink on the surface, when the reflection from the inner surface of 
the glass is seen. These empirical tests are usually sufficient, and it 
is rarely necessary to resort to testing the specific gravity and hard- 
ness, which provide further means of identification. It is worthy 
of note, however, that such imitation pearls are unique amongst 
imitation gems in that, in some respects, they are actually superior 
to the natural article. They are considerably harder, for instance, 
and their luster is not affected by constant wear. 
In conclusion, I would like to refer very briefly to the present 
position of gems from the economic point of view. It is, perhaps, 
natural that the considerable influx of artificial gems in recent years, 
more particularly of the corundum species, has led to a great deal 
of controversy and difference of opinion as regards their merits. On 
the one hand the vendors of the artificial stones often publish ex- 
travagant statements as to their defying identification, which, as I 
have shown you, is all nonsense. On the other hand, those interested 
in maintaining the prestige of the natural article make equally un- 
reasonable statements, to the effect that such artificial productions, 
