Mineral Species. We must first consider what is meant by a "mineral species" and find out what relation exists between that subject and chemical composition. Now by a "mineral species" is understood a single substance, having (except for mechanically admixed impurities) practically a constant chemical composition, and having practically identical physical properties in all specimens of it.
Diamond and Corundum. A chemist would call a true mineral a pure substance, just as sugar and salt are pure substances to the chemist. Thus diamond is a "mineral species," as is also corundum. There are many different colors of both diamond and corundum, but these different colors are believed to be due to the presence in the pure substance of impurities in small amounts. Thus every diamond consists mainly of pure carbon, and all the corundum gems (ruby and the various colors of sapphire) consist mainly of pure oxide of aluminum. The properties of all diamonds are practically alike and so are the properties of all the corundum gems whether red (ruby), blue (sapphire), yellow (Oriental topaz), green (Oriental emerald), or purple (Oriental amethyst).
Thus all diamonds, of whatever color, belong to the one species, diamond, and in this case the usual custom in naming them agrees with the facts. Similarly all sapphires, of whatever color, belong to the mineral species "corundum." Thus a ruby is a red corundum.
The old French traveler and gem merchant, Tavernier, tells us that in the seventeenth century, when he visited the mines of Pegu, the natives knew of the similarity of the corundum gems and even called all by one name, with other names attached to designate the color. Singularly enough, the common name used by them was ruby rather than sapphire, as now. Thus they called blue corundum gems blue rubies; yellow corundums, yellow rubies, etc.
It is easily seen that if one recognizes the similar nature of all the many colors and shades of corundum that the number of things that one has to remember in order to be well acquainted with these stones is considerably diminished. Thus, instead of having a whole series of specific gravities to remember one has only to remember that all the corundum gems have a specific gravity of approximately 4. Similarly they are all of practically the same refractive index (1.761-1.770, being doubly refracting) that they all exhibit dichroism when at all deeply colored, etc.
Having thus indicated what we mean by mineral species and having illustrated the matter by the cases of diamond and corundum and further having stated that all diamonds are composed of pure carbon (except for traces of impurities) and all corundum gems mainly of oxide of aluminum, we may proceed to consider other mineral species and find out what gems they afford us.
Carbon, the only Element Furnishing a Gem. It will be noted that the first species considered, diamond, consisted of but a single element, carbon. It is thus exceedingly simple in composition, being not only a pure substance but, in addition, an elementary substance. Corundum, the second species considered, was a little more complex, having two elements, aluminum and oxygen, in its make-up, but completely and definitely combined in a new compound that resembles neither aluminum nor oxygen. It is thus a compound substance. No other element than carbon affords any gem-stone when by itself.
Oxides of Metals. There is, however, another oxide, in addition to aluminum oxide, that furnishes gem material. It is silicon oxide, containing the two elements silicon and oxygen. Silicon itself is a dark, gray, crystalline element that seems half metallic, half non-metallic in its properties. It is never found by itself in nature but about twenty-eight per cent. of the crust of the earth is composed of it in compound forms, and one of the most abundant of these is quartz, which is a mineral species, and which contains just silicon and oxygen. That is, it is oxide of silicon. Now quartz is colorless when pure (rock crystal), but it is frequently found colored purple (probably by oxide of manganese) and it is then called amethyst by the jeweler. At other times its color is yellow (due to oxide of iron) and then the jeweler is prone to call it "topaz," although properly speaking that name should, as we shall soon see, be reserved for an entirely different mineral species. Chalcedony too (which when banded furnishes us our agates, and when reddish our carnelian) is a variety of quartz, and prase is only quartz colored green by fibers of actinolite within it.
The common cat's-eye and the tiger's-eye are varieties of quartz enclosing fibrous minerals or replacing them while still keeping the arrangement that they had. "Venus hair stone" is quartz containing needle-like crystals of rutile, and "iris" is quartz that has been crackled within, so as to produce rainbow colors, because of the effects of thin layers of material. Aventurine quartz (sometimes called goldstone) has spangles of mica or of some other mineral enclosed in it. The jaspers are mainly quartz with more of earthy impurity than the preceding stones.
Thus all this long list of stones of differing names can be classified under the one mineral species, quartz. Together they constitute the quartz gems. In properties they are essentially alike, having specific gravity 2.66, hardness 7, slight double refraction, etc., the slight differences that exist being due only to the presence of varying amounts of foreign matter.