CHAPTER XV. COLOURED PEARLS.
Brighter the offspring of the morning dew,
The evening yields a duskier birth to view ;
The younger shells produce a whiter race,
We greater age in darker colours trace.”
—Marbodus.
IΝ the above quotation from the curious old ” Lapidarium ” of Marbodus. following the translation given by the Rev. C. W. King, we have an exposition in brief of the views of the ancients respecting the cause of the various tints which are assumed by certain Pearls. It was commonly held by the early naturalists, that the dark-coloured Pearls had been formed either under the gloomy influence of the shades of evening, or by an aged oyster.
Dismissing these fancies, however, we may refer to the opinion so often expressed and still entertained in some quarters, that the black colour of a Pearl is traceable to some disease in the Pearl-bearing mollusc. Although the origin of the colour is in the deepest degree obscure, it seems probable that it is in some cases due to the presence of certain pigments in the medium in which the molluscs live. The subject of the colouring matter of the nacre in the shells of the genus Unto, afforded matter for an interesting discussion at the meeting of the Academy of Natural Sciences of Philadelphia, on March 20, 1860.
If we know the nature of the pigment which colours the nacreous lining of the shell, we may safely conclude that we know also the character of the colouring matter in the tinted Pearl ; inasmuch as a Pearl is of precisely the same nature as the nacre of its shell.
Here we refer not to the pearly hue of a nacreous shell, which, as explained in an early chapter (p. 87), is a purely optical phenomenon, but to the substantive colour of the carbonate of lime which constitutes both the nacre and the Pearl, and which colour is, no doubt, due to the presence of some material pigment.
The late Dr. James Lewis, of Mohawk, New York, suggested that the colour of many freshwater shells might be caused by certain salts of gold, minute quantities of the precious metal, probably in the state of a chloride, being held in solution in the stream, and received into the system of the mollusc. ” I notice,” said Dr. Lewis, “that colours are most brilliant in regions where gold may be suspected. In the lake regions of the western States, minerals are abundant, and the conditions are not incompatible with the supposition that gold is sparingly disseminated among them, in quantities too small perhaps, to be available. . . .
Of two streams producing identically the same species, one will give a large proportion of white nacres, and the other will present coloured nacres ; and usually we also notice another phenomenon — a greater brilliancy of nacre where rich colours abound. In this case I have my private opinion that gold produces its peculiar tonic effect, for tonic it is, under certain circumstances, by increasing the secretion.”
Dr. Isaac Lea, the great authority on the genus Unio, was not disposed to attribute the colour to any auriferous compound ; and remarked that Dr. Draper had calcined purple shells, and could not detect in the white calx any metallic substance. According to Dr. Lea, the colours of many of the American Unionidee with purple, pink, and salmon-coloured nacre, are probably due to the presence of some organic compound, such as is supposed to produce the colour of certain kinds of fluor-spar and quartz.
It is a curious fact that the Unto rectus is usually white in the Ohio river, while in the more northern waters it is generally of a fine rich purple or salmon colour. Again, the Unio ligamentinus has probably never been found, either pink or purple in the Ohio, while at Grand Rapids, in Michigan, the pink and salmon shells are quite common.
In like manner the Margaritana margaritifera—or common Pearl-mussel—has usually a fine purple nacre in the Columbia river and its tributaries, while it is almost universally white in the rivers of Pennsylvania, Connecticut, and Massachusetts, as is also the case in Northern Europe. Instances like those seem to prove that the colour is caused by certain foreign substances, which are present in some streams and not in others.
It seems probable that the gold theory of Dr. Lewis may explain the origin of the purple and violet colours of certain Pearls, inasmuch as it is well known that such tints are readily yielded by salts of gold : thus the magnificent dye termed “purple of Cassius” is a compound of the chlorides of gold and tin. According to the American conchologist, Mr. Tryon, jun., the Pearls found occasionally in Attornia cepa are purple, while those in Arca NOCE are violet. These, however, are of no commercial value.