Leisure hours among the gems

by Augustus C. Hamlin

This mineral has not only been an object of delight to the fashionable world, but it has also been a wonder and a perplexity to the philosophers. In admiring its beauties and attempting to account for its phenomena, Newton was led to the series of experiments and to that train of sublime reasoning that gave to science the most brilliant and extraordinary of his discoveries. The colored refractions of the gem reminded the philosopher of the iridescence of the soap-bubble, and the soap-bubble suggested the undulatory theory of light. Newton, after long study of the opal, is said to have declared that its hues were produced by the refractions and reflections of light at the numerous minute fissures which traverse the stone in all directions. But this theory is denied by many at the present day, and especially by Mohrs, who maintains that the thin films of air filling the cavities of the stone would produce iridescence only. Other opticians believe the colored reflections to be due to laminæ formed by incipient crystallization, as seen in the equally remarkable mineral known as labradorite. Babinet believes the brilliant colors of the opal to be due to the narrow fissures in the stone, like those produced in the partial fracture of glass or quartz. He also refers for example to the colors of thin transparent plates, and believes that the colors of flowers are produced in like manner from the overlaying of the transparent tissues of which the petals are composed. This, then, according to the French philosopher, is the secret of the gorgeous hues of vegetation from their first development to the period of their final decay. The diamond, when cut in a regular form, displays the most magnificent flashes of the prismatic hues by artificial light; and although the mineral is composed of an infinite number of laminæ, no one maintains the theory that the color is produced by thin films of air like those in the soap-bubble. We also may observe the same hues sparkling among the dew-drops in the morning sunlight, and likewise in the artificial diamonds, which are composed of solid glass and apparently homogeneous.

In examining the interior of an opal, we often fail to perceive any cause for the reflections of color, especially in the limpid varieties. The flashes appear when the light enters the stone at a certain angle, but when viewed in any other direction the gem presents the usual appearance of common transparent quartz. In other varieties of the mineral, however, especially the milky or translucent, a cause for the colored reflections is easily observed. We have under observation the beautiful opal known as the “Oberon,” and beneath its translucent surface appear thin films of a faint reddish hue suspended at different depths within its interior. They are so well defined that their edges may be recognized, and they lie like thin clouds suspended in a hazy sky. But as the gem is turned so that the light strikes the film at a different angle, the scene is instantly changed, and a mass of flame replaces the sombre tint. It is a little curious that some of the films exhibit the different colors of the spectrum as the angle of light is changed, while others display only the green and blue color, no matter how the light strikes them. In fact, the films or patches, which are apparently alike, produce different results from the same rays of light; and some display the continuous spectrum, while others exhibit but one color. It is a little singular that all transparent minerals when fractured do not exhibit alike in their fractures the prismatic gleams displayed by quartz and glass. We have before us a beautiful transparent white crystal of adularia or moon-stone from St. Gothard, and although it is fissured and fractured in a thousand places, yet we observe little iridescence in it even when exposed to the sunlight.

The localities where the precious opal is now found are but few, and none of them were probably known to the ancients. All record of the old opal mines is now lost; but there were undoubtedly deposits of the mineral in Arabia, Syria, and in Asia, whence the ancients derived their gems. The famous Hungarian mines were not discovered until late in the fifteenth century, and the country was quite unknown to the Romans.

The principal mines explored at the present day, and whence most of our opals are now derived, are those of Hungary and Honduras. The Hungarian mines are of great extent and are now scientifically explored, but those of Central America are undetermined and but rudely mined. It is believed that there are other mines in Central America besides those of Honduras, for the natives at times bring fine specimens to the coast from localities widely separated. It is quite true that most of the opals of America are less hard than the Hungarian, but they are no less brilliant, and some of them withstand atmospheric effects and the wear of time quite as well. The Honduras opals are found near Gracias a Dios in porcelain earth, and are extracted in irregular masses, sometimes uniform or globular concretions, with rough and deeply indented surfaces. These masses do not exhibit the least tendency to crystallization like quartz, and they are generally quite small. Their natural colors are pale, and vary from brown to a pearly gray. They often exhibit a rich and varied play of the rainbow hues, even in their natural and rough condition. But sometimes, when this rough exterior is removed by the lapidary’s wheel, and the gem is highly polished, the colors vanish as if by magic. The polished stone no longer displays a single ray of the brilliant fires which illuminated every angle of the stone when in its rough state. This singular disappearance may be explained by the theory that the surface has been too highly polished, and the substance of the stone is rendered too transparent to permit the requisite degree of reflection, for when the surface is slightly roughened the play of colors again returns. The finest specimens are therefore those which are translucent, or those which, being transparent, are backed by an opaque ground which refracts the light.

The opal-bearing districts in Central America are far more extensive than is generally supposed. The Province of Honduras abounds in them, and we have evidence of others occurring in the State of Guatemala on the Pacific coast. The following descriptions of some of the opal mines of Honduras were published by Dr. J. Le Conte, in 1868, in his report of the Inter-oceanic Railroad survey:—

“Extensive beds of common opal and semi-opal are seen along a belt extending through the central part of the department of Gracias; but these varieties, though very beautiful and possessing high interest to the mineralogist, are without commercial value from the ease with which artificial products may be made which precisely resemble them. The localities worthy of exploration are those in which the opal forms veins (not beds) in compact but brittle trachyte of a dark color. The veins, as will be seen, are not confined to such rock, but seem to have their origin in it, and are probably not found except in connection with it. The best-known mines of precious opal are in the department of Gracias; several localities have yielded valuable gems, but they are all remote from the line of road. Some are in the vicinity of the town of Gracias, others near Intibucat; but the most important are at Erandique. The working is now carried on in a very small way; but the locality is extensive, and in my opinion mining on a large scale would be attended with profit. The country near by abounds with beds of common opal, as in many other places, but the gems occur in somewhat irregular veins running in a northeast and southwest direction, and with a nearly perpendicular dip. The veins are not continuous, but branch off and disappear at short intervals; neither are the contents of uniform quality, but the valuable parts are usually in belts in the vein, and limited on each side by portions of ordinary opal without play of colors. These lines of light are sometimes numerous and narrow, alternating with the common opal forming a very beautiful gem. Many again, even of large size, are uniform in structure, and exhibit a play of colors as brilliant as the finest opals from Hungary. The hill in which they are found is about two hundred and fifty feet high, and two or three miles in length, and for a width of half a mile for its whole length opals have been found wherever excavations have been made. The rock in which they occur is a hard, brittle trachyte of a vitreous lustre, and splintering into acute fragments when struck; a bed several feet in thickness overlying this rock is of a gray color and soft consistence, and also contains opal veins; it is probably a trachyte changed by atmospheric action.

“Other localities within two leagues of Erandique have furnished very fine opals, but as they are not now worked I did not visit them. Many places on the road between Intibucat and Las Piedras appear favorable to the existence of opal mines; but only careful scrutiny by a number of explorers can discover them. I would mention as most worthy of future attention the vicinity of Lepasale and of Yucusapa and the ascent of the great mountain of Santa Rosa. Greater expectations and indeed almost certain success will attend the search for opal mines in the valley leading from Tambla towards the pass of Guayoca, nearly on the line of the proposed road. Within half a mile of Tambla are immense beds of common opal of various shades of color. Near Guayoca are banded opals of alternate layers of opaque and semi-transparent white, having the appearance of onyx; these occur in a red vitreous trachyte and sometimes in contact with the masses of petrified wood which strew the ground for a considerable distance. Veins of a pearl-colored opal, with red reflections, are also found here; they have no commercial value, but serve as indications of better things in the neighborhood.

“Between the two localities mentioned (that near Tambla and that of Guayoca), Mr. W. W. Wright, chief assistant of the survey, has, by following some obscure indications, arrived at a vein of very pretty glassy opals and yellow fire opals, not of great value, but serving to strengthen the opinion expressed of the ultimate discovery of precious opals in the vicinity. Near Choluteca are found fire opals, some of which I was told possess merit. One (not of the best) given me is precisely similar to those obtained by Mr. Wright near Tambla. Within one league of Goascoran, as I am informed by Fernando Gaillardo, a resident of that town, is a mine producing opals with a good play of colors.”

Another remarkable deposit of opal was found by Mr. Wright about five miles east of Villa San Antonio in the plains of Camayagua. Though not of high value, it may be of use for ornamental purposes, being of a fine red color with transparent amethystine bands. It occurs in veins in gray porphyry, sometimes several inches thick, and may be procured in large quantities. Precious opal has been discovered in the iron mines at Barcoo in Queensland, and a number of specimens were exhibited at Philadelphia, at the Centennial. Some of these specimens were very fair, and gave promise of choice gems. The blue tints displayed by some of them were of great purity. They appeared to be of the hard variety, and therefore less liable to be affected by the ravages of time, or influence of exposure.

We will not fatigue our readers with a long dissertation on the formation of the opal. We will however, quote one theory which all may understand.