1825.] Mr. Gray on the Structure of Pearls. 27 



Article IV. 



On the Structure of Peails, and on the Chinese Mode of produc- 

 ing them of a large Size and regular Form. By John Edward 

 Gray, MGS. 



(To the Editors of the Annals of Philosophy.) 



GENTLEMEN, Dec. 10, 1824. 



Pearls are merely the internal pearly coat of the shell, which 

 has assumed, from some extraneous cause, a spherical form ; 

 they are, like the shell, composed of concentric coats formed of 

 perpendicular fibres ; consequently when broken they exhibit con- 

 centric rings and fibres radiating from a central nucleus usually 

 consisting of a grain of sand or some other body which has irritated 

 the animal. A pearl having been once formed, the animal con- 

 tinues to increase its size by the addition of fresh coats, perhaps 

 more rapidly deposited on it than on the rest of the shell, as the 

 prominence remains a source of irritation. 



The pearls are usually of the colour of the part of the shell to 

 which they are attached. I have observed them white, rose 

 coloured, purple,* and black, and they are said to be sometimes 

 of a green colour ; they have also been found of two colours, 

 that is, white with a dark nucleus, which is occasioned by their 

 being first formed on the dark margin of the shell before it is 

 covered with the white and pearly coat of the disk, which, when 

 it becomes extended over them and the margin, gives them that 

 appearance. 



Pearls vary greatly in their transparency. The pink are the 

 most transparent, and in this particular they agree with the 

 internal coat of the shell from which they are formed, for these 

 pearls are only formed on the pinnae, which internally are pink 

 and semitransparent, and the black and purple specimens are 

 generally more or less opaque. 



Their lustre, which is derived from the reflection of the 

 light from their peculiar surface produced by the curious 

 disposition of their fibres, and from their semitransparency and 

 form, greatly depends on the uniformity of their texture and 

 colour of the concentric coats of wiiich they are formed. That 

 their lustre does depend on their radiating fibres may be distinctly 

 proved by the inequality of the lustre of the "Columbian pearls" 

 which are filed out of the thick part near the hinge of the pearl 



* I can witli certainty infonn the anonymous author in the Edinburgh Philosophical 

 Journal, No. xxi.p. 41, who observes, that " in the Bvitish JMuseuui there is or uas a 

 famous pink pearl," that there not only now is one, but three of these pearls, as he 

 might have convinced himself, for they have been exposed to the public now for these 

 last three or four years to uiy own knowledge. 



