A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public Part 8

Having thus indicated what we mean by mineral species and having ill.u.s.trated 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 FURNIs.h.i.+NG 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 p.r.o.ne 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 cla.s.sified under the one mineral species, quartz. Together they const.i.tute 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.

OPAL. The _opal_ may be considered along with the quartz gems, because, like them, it is composed mainly of oxide of silicon, but the opal also has water combined with the silicon oxide (not merely imprisoned in it).

Thus opal is a hydrous form of silica (hydrous comes from the Greek word for water).

SPINEL. All our other stones are of more complicated chemical composition than the preceding. Coming now to mineral species which have three chemical elements in them we may consider first _spinel_, which has the two metallic elements aluminum and magnesium and the non-metallic element oxygen in it. It is virtually a compound of the two oxides, aluminum oxide and magnesium oxide. The variously colored spinels, like the various corundums, all have the same properties, thus they are all of hardness 8 or a little higher, they all have single refraction, and all have specific gravity 3.60.

CHRYSOBERYL. Another mineral species which, like spinel, has just three elements in its composition is _chrysoberyl_. This mineral contains the metals aluminum and beryllium combined with the non-metal oxygen. Thus it is really to be regarded as a compound of the two oxides, aluminum oxide and beryllium oxide. This species furnishes us _Alexandrite_, _chrysoberyl cat's-eye_ and less valuable chrysoberyls of yellowish-green color. All are of the one species, the marked color difference being due to the presence of different impurities. The cat's-eye effect in one of the varieties is due to the internal structure rather than to the nature of the material.

THE SILICATES. Nearly all of the remaining precious stones belong to a great group of mineral species known as the silicates. These are so called because they consist largely of oxide of silicon (the material above referred to under quartz gems). This oxide of silicon is not free and separate in the silicates but is combined chemically with other oxides, chiefly with metallic oxides. Thus there are many different silicates because, in the earth, many different metallic oxides have combined with silicon oxide. Also in many cases two or three or even more metallic oxides have combined with silicon oxide to make single new compounds.

GLa.s.s, A MIXTURE OF SILICATES. Those who are familiar with gla.s.s making may receive some help at this point by remembering that the various gla.s.ses are silicates, for they are made by melting sand (which is nearly pure oxide of silicon) with various metallic oxides. With lime (calcium oxide) and soda (which yields sodium oxide) we get soda-lime gla.s.s (common window gla.s.s). Lead oxide being added to the mixture a dense, very brilliant, but soft gla.s.s (flint gla.s.s) results. Cut gla.s.s dishes and "paste" gems are made of this flint gla.s.s. Now the gla.s.ses, although they are silicates, are not crystalline, but rather they are _amorphous_, that is, without any definite structure. Nature's silicates, on the other hand, are usually crystallized or at least crystalline in structure. (In a few cases we find true gla.s.ses, volcanic gla.s.s, or obsidian, for example.)

Having thus introduced the silicates we may now consider which ones among the many mineral silicates furnish us with precious or semi-precious stones.

BERYL, EMERALD, AND AQUAMARINE. First in value among the silicates is _beryl_, which, when gra.s.s green, we call _emerald_. The _aquamarine_ and _golden beryl_ too belong to this same species. Beryl is a silicate of aluminum and beryllium. That is, it is a compound in which oxide of silicon is united with the oxides of aluminum and of beryllium. There are thus four chemical elements combined in the one substance and it is hence more complicated in its composition than any of the gems that we have yet considered. It is worthy of note that aluminum occurs in the majority of precious stones, the only species so far considered that lack it being diamond, and the quartz gems.

Perhaps the silicates that are next in importance to the jeweler, after beryl, are those which form the _garnets_ of various types. There are four princ.i.p.al varieties of garnet (although specimens of garnet frequently show a crossing or blending of the types).

GARNETS. The types are (1) _Almandite_ garnet; (2) _Pyrope_ garnet; (3) _Hessonite_ garnet; and (4) _Andradite_ garnet. These are all silicates, the almandite garnets being silicates of iron and aluminum; the pyrope garnets are silicates of magnesium and aluminum; the hessonite garnets, silicates of calcium and aluminum, and the andradite garnets, silicates of calcium and iron.

The so-called almandine garnets of the jeweler are frequently of the almandite cla.s.s and tend to purplish red. The pyrope garnets are, as the name literally implies, of fire red color, as a rule, but they also may be purplish in color. The hessonite garnets are frequently brownish red and are sometimes called "cinnamon stones." The andradite garnets furnish the brilliant, nearly emerald green demantoids (so often called "_olivine_" by the trade).

Thus all the garnets are silicates and yet we have these four princ.i.p.al mineral species, which, however, are more closely related to each other in crystal form, in character of composition and in general properties, than is usual among the other silicates. Specimens which have any one of the four types of composition unblended with any of the other types would be found to be exactly alike in properties. As was suggested above, however, there is a great tendency to blend and this is well ill.u.s.trated by the magnificent _rhodolite_ garnets, of rhododendron hue which were found in Macon County, North Carolina. These had a composition between almandite and pyrope, that is, they had both magnesium and iron with aluminum and silica.

The true TOPAZ next calls for consideration as it too is a silicate. The metallic part consists of aluminum, and there are present also the non-metals fluorine and hydrogen. Here we have five elements in the one substance. Various specimens of this species may be wine yellow, light blue, or bluish green, pink or colorless, yet they all have essentially the same properties.

TOURMALINE is about as complicated a mineral as we have. It is a very complex silicate, containing aluminum, magnesium, sodium (or other alkali metal, as, for example, lithium), iron, boron, and hydrogen. As Ruskin says of it in his _The Ethics of the Dust_, when Mary asks "and what is it made of?" "A little of everything; there's always flint (silica) and clay (alumina) and magnesia in it and the black is iron, according to its fancy; and there's boracic acid, if you know what that is: and if you don't, I cannot tell you to-day and it doesn't signify; and there's potash and soda; and on the whole, the chemistry of it is more like a mediaeval doctor's prescription, than the making of a respectable mineral." The various tourmalines very closely resemble each other in their properties, the slight differences corresponding to differences in composition do not alter the general nature of the material.

MOONSTONE belongs to a species of mineral known as feldspar. The particular feldspar that furnishes most of the moonstone is orthoclase, a silicate of pota.s.sium and aluminum. Another feldspar sometimes seen as a semi-precious stone is _Labradorite_. _Amazonite_, also, is a feldspar. _Sunstone_ is a feldspar which includes tiny flakes or spangles of some other mineral.

The mineral species _olivine_ gives us _peridot_. It is a silicate of magnesium.

ZIRCON is itself a species of mineral and is a silicate of zirconium.

The names _hyacinth_, _jacinth_, and _jargoon_ are applied to red, yellow, and colorless zircon in the order as given.

JADE may be of any of several different species of minerals, all of which are very tough. The princ.i.p.al jades belong, however, to one or the other of two species, _jadeite_ and _nephrite_. Jadeite is a sodium aluminum silicate and nephrite, a calcium magnesium silicate.

Leaving the silicates we find very few gem minerals remaining. The phosphates furnish us _turquoise_, a hydrous aluminum phosphate, with copper and iron. _Variscite_ is also a phosphate (a hydrated aluminum phosphate).

The carbonates give us _malachite_ and _azurite_, both carbonates of copper with combined water, the malachite having more water.

LESSON XIX

THE NAMING OF PRECIOUS STONES

Owing to the confusion which may result from a lack of uniformity in the naming of precious stones, it is very desirable that jewelers and stone merchants inform themselves in regard to the correct use of the names of the gems, and that they use care in speaking and in writing such names.

As nearly all precious and semi-precious stones are derived from a relatively small number of _mineral species_, as we saw in Lesson XVIII., and as the science of _mineralogy_ has a very orderly and systematic method of naming the minerals, the best results are had in the naming of gems when we use, as far as is possible, the language of mineralogy.

ANCIENT USAGE. Long established custom and usage, however, must be observed, for any system of naming must be generally understood in order to be useful. Thus the proper name for blood red, crystallized oxide of aluminum, of gem quality, according to the mineralogical system of naming, would be red _corundum_, but that same material is referred to in the Old Testament thus (in speaking of wisdom), "She is more precious than _rubies_." It is obviously necessary to keep and to use all such terms as have been for years established in usage, even though they do not agree with the scientific method of naming the particular mineral.

It is, however, necessary that any name, thus retained, should be correctly used, and that it should not be applied to more than one material. Thus the term _ruby_ should be reserved exclusively for red corundum, and not applied to other red minerals such as garnet, spinel, etc., as is too often done.

It will be the purpose of this lesson to attempt to set forth as clearly and as briefly as possible what const.i.tutes good usage in the naming of the princ.i.p.al stones, and also to point out what incorrect usage is most in need of being avoided.

To cover the subject systematically we will adopt the order of hardness that we did in discussing mineral species in Lesson XVIII.

FANCY DIAMONDS. Beginning with the hardest of all gems, the _diamond_, we have no difficulty as regards naming, as all specimens of this mineral, regardless of color, are called diamonds. When it is necessary to designate particular colors or tints, or differences in tint, additional names are used--for example, all diamonds of p.r.o.nounced and pleasing color are called "fancy" diamonds in the trade. Certain of these "fancy" diamonds are still further defined by using a name specifying the color, as, for example, "canary" diamonds (when of a fine bright yellow), or "golden fancies," when of a fine golden brown, or "orange," or "pink," or "absinthe green," or "violet," as the case may be.

NAMES OF VARIOUS GRADES OF WHITE DIAMONDS. The great majority of the diamonds which come on the market as cut stones belong, however, to the group which would be spoken of as white diamonds, but many qualifying names are needed to express the degree of approach to pure white possessed by different grades of these diamonds. Thus the terms: 1, _Jagers_; 2, _Rivers_; 3, _Blue Wesseltons_; 4, _Wesseltons_; 5, _Top Crystals_; 6, _Crystals_; 7, _very light brown_; 8, _Top Silver Capes_; 9, _Silver Capes_; 10, _Capes_; 11, _Yellows_, and 12, _Browns_, describe _increasing_ depth of color, and hence _decreasing_ value in diamonds.

POPULAR NAMES. Certain more popular names for diamonds of differing degrees of whiteness may next be set forth. The term "blue white" (a much abused expression, by the way) should be applied only to diamonds of such a close approach to pure whiteness of body substance, as seen on edge in the paper that, when faced up and undimmed, they give such a strong play of _prismatic_ blue that any slight trace of yellow in their substance is completely disguised, and the effect upon the eye is notably blue. This would be the case with stones of the grades from 1 through 4 in the list above. Grades 5 and 6 might properly be called "_fine white_," and grades 7, 8, and 9 simply "_white_." Grade 10 is frequently spoken of as "_commercial white_," and grade 11 sometimes as "off color." Grade 12 includes all degrees of brownness except the very light shades and the deep, pretty shades of the "fancy" browns.

RUBIES. Leaving the naming of the different colors of diamonds we come to the gems furnished us by the mineral known as _corundum_. As we have previously seen, this mineral occurs in many different colors and with wide differences of tint and shade in each of the princ.i.p.al colors. The best practice with regard to naming the corundum gems is to call the red material, when of a good, full red of pleasing shade, _ruby_. The finest shades of blood red are usually called "_Burmah rubies_" because more rubies of this quality are found in Burmah than anywhere else. Any ruby of the required shade would, however, be called a Burmah ruby in the trade regardless of its geographical origin. The most desirable tint among Burmah rubies is that which is known as "pigeon blood" in color.

This color is perhaps more accurately defined as like the color in the center of the red of the solar spectrum. Certain slightly deeper red rubies are said to be of "beef blood" color. The English are said to prefer these. Those of slightly lighter tint than pigeon blood are sometimes referred to as of "French color," from the fact that they are preferred by French connoisseurs.

Rubies of dark, garnet-like shade are known as "_Siam rubies_," many such being found in that country. Light pinkish rubies are called "_Ceylon rubies_." It should be clearly kept in mind that all these "rubies" are of red corundum, and that in all their distinctive properties except color they are essentially similar.

SAPPHIRES. Corundum of fine blue color is known as "_sapphire_." The "cornflower blue" seems to be most in favor at present. Such sapphires are sometimes called "_Kashmir sapphires_" because many fine ones come from that State. "_Ceylon sapphires_" are usually paler than the cornflower blue. "_Montana sapphires_" are usually of greenish blue or pale electric blue. Such fine blue stones as are mined in Montana would be sold under another name according to the quality of their color, and not as "Montana sapphires." "_Australian sapphires_" are of a very deep, inky blue, and do not command a high price. Here again, as with rubies, the cla.s.sification depends upon the color rather than upon the origin, although the geographical names that are used, correctly state the usual source of stones of the particular color.

All corundums other than ruby and blue sapphire are usually called by the term "sapphire," with a qualifying adjective designating the color; thus we may have pink sapphire, golden sapphire, green sapphire, etc.

When of very fine yellow color the yellow sapphire is sometimes called "_Oriental topaz_" by jewelers, the term "_Oriental_" as thus used indicating that the material is corundum. We also have "_Oriental amethyst_" and "_Oriental emerald_" for the purple, and the fine green, and "_Oriental aquamarine_" for the light blue-green corundum. The yellow corundum is also sometimes called "_King topaz_," especially in Ceylon. Inferior sapphires of almost every conceivable color are frequently a.s.sorted in lots and sold as "fancy sapphires." Such lots, however, almost always need recla.s.sification as they often contain as many as a dozen mineral species besides corundum.

Sapphires and rubies of minute tubular internal structure frequently display a beautiful six-pointed star when cut to a round-topped cabochon shape and exposed to direct sunlight or to light from any other single source. Such stones are named "_star sapphire_" and "_star ruby_."

The artificial rubies and sapphires should all be called _scientific ruby_ or _sapphire_, and not "_reconstructed_" or "_synthetic_" as none are made to-day from small, real rubies, and as the process is in no sense a chemical synthesis.

CHRYSOBERYL. Leaving the corundum gems we come next to chrysoberyl. When the gems furnished by this mineral are of a fine green by daylight, and of a raspberry red by artificial light, as is sometimes the case, they should be called "_Alexandrites_" (after the Czar Alexander II., in whose dominions, and on whose birthday, the first specimens are said to have been discovered). When chrysoberyl is of fibrous or tubular internal structure it affords cat's-eyes (when cabochon cut), and these should be specifically named as "_chrysoberyl cat's-eye_" to distinguish them from the less beautiful and less valuable quartz cat's-eyes. Other varieties of chrysoberyl (most of those marketed are of a greenish-yellow color) are correctly named simply "_chrysoberyls_." Such stones are, however, sometimes incorrectly called "_chrysolite_" by the trade, and this practice should be corrected, as the term chrysolite applies correctly only to the mineral _olivine_ which gives us the _peridot_.

SPINEL. Next in the order that we have chosen comes "_spinel_." The more valuable spinels are of a red color that somewhat closely approaches the red of some rubies. Such red spinels should be called "_Ruby spinel_"

(and _not spinel ruby_). The stones themselves sometimes get mixed with corundum rubies (they are frequently found in the same gem gravels), and this makes it all the more necessary that both stones and names should be clearly distinguished. Some dealers call reddish spinels "_Balas ruby_" (rose red), and orange red ones "_rubicelle_." Violet red spinel is sometimes called "_almandine spinel_." It is very desirable that the name of the mineral species, _spinel_, should be used, together with a qualifying color adjective, in naming gems of this species, rather than such terms as "rubicelle," "balas ruby," "spinel ruby," etc.

TOPAZ. We come now to _topaz_. True, or _precious topaz_, as it is usually called, to distinguish it from the softer and less valuable yellow quartz, is seldom seen in the trade to-day. Jewelers almost always mean yellow quartz when they speak of "topaz." This is an unfortunate confusion of terms, and one which will be hard to eradicate.

There is seldom any injustice done through this misnaming, as the price charged is usually a fair one for the material offered. Considerably higher prices would be necessary if true topaz was in question.

An instance from the writer's experience will serve to ill.u.s.trate the confusion that exists in the trade as to what should be called topaz. A jeweler of more than ordinary acquaintance with gems exhibited some fine brooch stones as specimens of topaz. On remarking that they were of course _citrine quartz_ rather than _true topaz_, the author was met with the statement that the brooch stones were _real_ topaz. In order to make clear to the dealer the difference between the two species, the author asked him if he hadn't some smaller topazes in stock that had cost him considerably more than the brooch stones. The dealer replied that he had some small wine yellow topazes for which he had paid more, and he produced them. The latter stones were true Brazilian topazes.

Most of them had tiny, crackly flaws in them, as is frequently the case, and, as the writer pointed out to the dealer, they had been bought by the _carat_, whereas the large brooch stones had been bought at a certain price per _pennyweight_. In fact the little stones had cost more per carat than the larger ones had per pennyweight.