Those who have had no practical acquaintance with minerals have little idea how variable and accidental are their colors. They may scarcely realize that the ruby and the sapphire are the same mineral, and that this mineral also occurs, and is used in jewelry, absolutely colorless, when it is known as lux sapphire, green as the so-called Oriental emerald, and yellow as the so-called Oriental topaz; that topaz itself may be yellow, brown, blue, or colorless; that zircons range from colorless through almost all conceivable shades of brown and green, and that even diamond has been found green, red and blue.
When we come to consider the properties by which precious stones are recognized, I shall say little or nothing about color, for it is of little value as a criterion. There are, for example, certain red stones which the most skillful experts cannot by their color alone refer with certainty to ruby, garnet or spinel. It might be expected that a noteworthy difference in chemical composition would accompany this difference of color, or that the pigment could be ascertained by analysis. In reality this is scarcely ever the case. It is fairly certain that the emerald owes its color to the presence of chromium, but the variation in the analyses of precious stones cannot generally be attributed to anything indicated by the variation of color.
The chemical composition, though of great general importance in mineralogy, is of little practical value in the discrimination of precious stones, since it is usually impossible to sacrifice a sufficient quantity for chemical analysis. If we are dealing with a faceted stone, not even the smallest portion can be utilized, for fear of injuring it.
There is, however, one remarkable optical property, which is ultimately related to the chemical composition. As is well known, many substances possess the property of absorbing certain rays of light. When the solar spectrum produced by admitting ordinary daylight through a slit, and transmitting it through a prism, is passed through the glowing vapor of certain substances, particular rays of light are absorbed, and their absence from the emerging fight is manifested by corresponding dark bands in the spectrum. The instrument by which the observations are made is the spectroscope. It is well known to most people that the solar spectrum itself contains certain dark bands of this sort, which are produced by vapors that can be identified by the position of the bands in the spectrum; and thus it is possible to ascertain something regarding the chemical constitution of the sun and certain of the heavenly bodies. Now, a precisely similar effect is produced by certain elements if present in a mineral, by merely transmitting the light through a piece of it. Thus, transparent minerals which contain the rare element didymium betray the presence of that element as soon as they are viewed through a spectroscope by ordinary daylight; the spectrum is seen to be traversed by black bands in the green, which are quite characteristic.
Among gem stones there are two which possess this curious property. One is the variety of red garnet known as almandine, and the other is the jargoon. The almandine produces characteristic bands in the green and the jargoon in the red, green and blue portion of the spectrum. To see these remarkable absorption spectra, to which attention was first called, I think, by my friend, Prof. Church, it is not necessary to look through the stone, it is quite sufficient to place it in a strong light, and look at it through an ordinary pocket spectroscope; the light which enters the instrument consists largely of rays which have penetrated the stone, and been reflected from the facets at the back. These rays produce the absorption spectrum. In this way we are enabled to identify a jargoon or an almandine merely by looking at it. There is no test so simple or so easy of application. It is curious that the almandine, or iron aluminum garnet, is the only garnet which presents an absorptive spectrum, and it is not yet certain to what element the bands are due. In the case of jargoon, they are supposed to be caused by the presence of some uranium compound in the mineral. All the almandine garnets which I have examined, and nearly all the jargoons, show these characteristic absorption spectra.
By way of summary, I have thought it desirable to indicate the general characters of precious stones in a diagram, which exhibits some of their relationships and also some of their differences in a graphic manner.
Opal, which is a comparatively light mineral, has a low refractive power; zircon or jargoon is a heavy mineral, and has a high refractive power. Let now the refractive power of any mineral (as measured by its refractive index for yellow light) be represented by a corresponding length set off from left to right, and let its density (as measured by its specific gravity) be represented by a corresponding length measured downward. Fixing in this way a point corresponding to opal, and another representing the character of zircon, draw a straight line from the one to the other. It will then be found that the points which, by their position on the diagram, represent the specific gravity and refractive index of the various minerals will be very nearly upon this line; that is to say, as the refractive index of precious stones increases, so also does their density, and the two increase together in a remarkably regular manner.
It appears from this table that those minerals which, by their high refractive power, possess the greatest brilliancy, possess also the highest specific gravity or weightiness; that the precious stones are therefore all heavy minerals. There is also a rough general correspondence between these characters and the hardness of the stones; the brilliant heavy minerals are also generally speaking hard.
Two remarkable exceptions display themselves. Sphene lies far to the right of the position which it should occupy according to its specific gravity; it possesses an extraordinarily high refractive index, and is, therefore, an extremely brilliant gem stone. On the other hand, a glance at the scale of hardness shows that it is, unfortunately, one of the softest of the possible gem stones, and that in this respect it is not very well fitted for jewelry.