PEARLS — THEIR CHARACTER AND USES. 
325 
The shape too is veiy variable and may be spherical, ovoid, pyriform 
or iiTegular, perhaps dependent upon the character and shape of the 
nucleus around which the pearl is formed, although perfectly spherical 
pearls are affirmed to always originate by the enclosure of the irritating 
particle within an ampulla or cyst filled with an organic fluid, which 
undergoes lamellar condensation around the intrusive atom, the 
interstices of the chitinous layer becoming filled with a calcareous 
deposit. The delicate wall of the cyst afterwards becomes ruptured 
and the incipient pearl or concretion is freed, so that the periodic 
deposition of pearly substance can continue to be uniforndy and 
evenly applied. 
If a pearl lying loose in the tissues becomes lodged between the 
mantle and the shell it may become attached to the inner surface of 
the shell by the successive accretions of pearly matter, and thereby 
lo.se its spherical form and the lustrous iridescence characterizing 
free pearls. 
The most valuable and esteemed pearls are of a lustrous white, 
delicately tinged with azure or .salmon, or those of an ex(|uisite 
rose color. The iridescence, which gives to pearls their greatest 
charm, is due to the diffraction of the 
light by the partially transparent and 
minutely corrugated layers forming the 
exterior of the gem ; the thinner and 
more transparent these layers the more 
beautiful the lustre, the superiority of 
the Oriental pearls in this re.spect being 
the reason of their gTeater beauty and 
value compared with British pearls. 
Besides the corrugated furrowing, there are a number of fine dark 
lines T?Vo ii^ch apart, which run in different directions or 
from pole to pole, and may add to the lirstrous effect. 
l.'nlike precious stones, pearls, being animal products, tj^uickly fade 
and decay and only great care keeps them in good condition for any 
length of time; they will tarnish when worn next the .skin or even 
when enclosed in a jewel box. AVdien dim and lustreless, their bril- 
liancy can, however, be restored by “peeling,” a process skilfully 
practised by the Chinese, who use for this purpose a keen knife, files, 
pearl powder and a scrap of leather, the final polishing being given by 
the leaf of a particular plant. 
Fig. 617. — Surface of Pearl, 
highly magnified, showing the mi- 
nute undulations upon which the 
pearly lustre depends (after Tryon). 
