with Observations on their Structure and Colour. 43 
centre, a smaller sphere is often seen to occupy the central part 
I have broken many such, and have seen this sphere from the 
size of a pin's head to that of half the bigness of the pearl 
broken ; and as this spherical body or central pearl remains at- 
tached to one-half of the pearl so broken, its concave impression 
is seen in the corresponding half. 
Pearls, when broken, are seen to consist of coats of Very diffe- 
rent colours, and many of those which have a silvery lustre in 
their outer coat, consist of various shades of brown and yellow 
within ; but 1 do not recollect that, on the contrary, any pearl 
externally brown, yielded, on being broken, an internal silvery 
surface ; whence it would appear that the larger the pearl grew, 
it partook of the silvery coating which forms the internal lining 
of most shells, notwithstanding that the same pearl may, in the 
earlier periods of its formation, have contracted some other colour 
from its local vicinity to some part of the shell of a particular 
colour. We see also from hence how useless are our endeavours 
to restore a pearl whose external surface is either decayed or 
chipped off, as such repairs in those of the finest kind exhibit a 
discoloured surface, which reminds one of decayed teeth. Many 
such are to be seen among the Regalia of England, and parti- 
cularly in one of the Queen's Crowns ; and it frequently happens 
that the coats of pearl are more uneven and rough as we ap- 
proach the centre, as happens in other calculi, which may have 
originated in many irregular grains, whether of sand or smaller 
pearls cemented together. On breaking a pearl of the finest 
water, of those produced by the river mussel in Scotland, (the 
Unio margariiifera ), several of its internal coats were found 
equal in beauty to its natural one ; but, on breaking a fine ori- 
ental pearl, it appeared to consist of uniform materials to its 
centre, whence probably arises the superior lustre and opaline 
transparency of the latter. 
That this uniformity of substance is the real cause of this su- 
perior beauty in the finer pearls, is confirmed by the degree of 
transparency observable in the pearls of the Pinna marina al- 
ready quoted, which, in some of them, amounts to a half trans- 
parency more, and then shewing a darker point in their centre, 
and occasionally the contrary ; but these pearls, like other 
bodies, the nearer they approach to a uniformity of substance, 
