A Text-Book of Precious Stones for Jewelers and the Gem-Loving Public Part 7

Now for our final example. A red stone, cut in the form of a pear-shaped brilliant, was submitted to the writer for determination. It had been acquired by an American gentleman in j.a.pan from an East Indian who was in financial straits. Along with it, as security for a loan, the American obtained a number of smaller red stones, a bluish stone, and a larger red stone. The red stones were all supposed to be rubies. On examination of the larger red stone with a lens it was at once noted that the internal structure was that of _scientific ruby_.

TESTING OTHER STONES. Somewhat dashed by the announcement of this discovery the owner began to fear that all his gems were false.

Examination of the small red stones showed abundance of "silk," a peculiar fibrous appearance within the stone caused by its internal structure. The fibers were _straight_ and _parallel_, not _curved_ and _parallel_ as in synthetic ruby. Tiny bubbles of angular shape also indicated that the small stones were natural rubies. They exhibited dichroism and scratched topaz and it was therefore decided that they at least were genuine.

The pear-shaped brilliant which was first mentioned was of a peculiar, slightly yellowish, red color. It was very pellucid and free from any striae either of the straight or curved types. It had in fact no flaws except a rather large nick on one of the back surfaces near the girdle.

This was not in evidence from the front of the stone and had evidently been left by the Oriental gem cutter to avoid loss in weight while cutting the stone.

The peculiar yellowish character of the red color led us to suspect ruby spinel. The stone was therefore inspected with the dichroscope and found to possess no dichroism. The sunlight-card test, too, showed that the stone was singly refracting.

A test of the hardness showed that the material barely scratched topaz, but was attacked by sapphire. It was therefore judged to be a red spinel.

The large bluish stone which the gentleman acquired with the red stones proved to be iolite, sometimes called cordierite or water-sapphire (_Saphir d'eau_), a stone seldom seen in this country. It had marked dichroism--showing a smoky blue color in one direction and a yellowish white in another. The difference was so marked as to be easily seen without the dichroscope.

LESSON XVII

SUITABILITY OF STONES FOR VARIOUS TYPES OF JEWELS, AS DETERMINED BY HARDNESS, BRITTLENESS, AND CLEAVABILITY

HARD STONES NOT NECESSARILY TOUGH. As was suggested in the lesson on hardness there is prevalent in the public mind an erroneous belief that hardness carries with it ability to resist blows as well as abrasion.

Now that _it does not follow that because a precious stone is very hard, it will wear well_, should be made plain. Some rather hard minerals are seldom or never used as gems, in spite of considerable beauty and hardness, because of their great brittleness. Other stones, while fairly hard and reasonably tough in certain directions, have nevertheless so p.r.o.nounced a cleavage that they do not wear well if cut, and are sometimes very difficult to cut at all.

In view of these facts it will be well to consider briefly what stones, among those most in use, are sufficiently tough as well as hard, to give good service in jewels, such as rings, which are subject to rough wear.

We may also consider those stones, whose softness, or brittleness, or ready cleavability, requires that they should be reserved for use only in those jewels which, because of their nature, receive less rough usage.

In order to deal with the princ.i.p.al gems systematically, let us consider them in the order of their hardness, beginning with the hardest gem material known, which is, of course, diamond.

DURABILITY OF THE DIAMOND. Fortunately this king of gems possesses in addition to its great hardness, considerable toughness, and although it is readily cleavable in certain directions it nevertheless requires a notable amount of force applied in a particular direction to cause it to cleave. Although sharp knocks will occasionally flake off thin layers from diamonds when roughly worn in rings, or even in extreme cases fracture them, yet this happens but seldom and, as the enormous use of the diamond in ring mountings proves, it is entirely suitable for that purpose. It follows that, if a stone can stand ring usage, it can safely be used for any purpose for which precious stones are mounted.

THE CORUNDUM GEMS. Next after the diamond in hardness come the corundum gems, _i. e._, ruby, sapphire, and the series of corundum gems of colors other than red and blue. These stones have no noticeable cleavage and are exceedingly tough, for minerals, as well as very hard. We have only to consider the use of impure corundum (emery) as a commercial abrasive in emery wheels, emery cloth, emery paper, etc., to see that the material is tough. Any of the corundum gems therefore may be used in any type of jewel without undue risk of wear or breakage. Customers of jewelers should, however, be cautioned against wearing ruby or sapphire rings on the same finger with a diamond ring in cases where it would be possible for the two stones to rub against each other. So much harder than the ruby is the diamond (in spite of the seeming closeness of position in Mohs's scale) that the slightest touch upon a ruby surface with a diamond will produce a p.r.o.nounced scratch. The possessor of diamonds and other stones should also be cautioned against keeping them loose in the same jewel case or other container, as the shaking together may result in the scratching of the softer materials. The Arabs are said to have a legend to the effect that the diamond is an _angry_ stone and that it should not be allowed to a.s.sociate with other stones lest it scratch them.

CHRYSOBERYL. Pa.s.sing on to the next mineral in the scale of hardness we come to chrysoberyl, which is rated as 8-1/2 on Mohs's scale. This mineral furnishes us the gem, alexandrite, which is notable for its power to change in color from green in daylight to red in artificial light. Chrysoberyl also supplies the finest cat's-eyes (when the material is of a sufficiently fibrous or tubular structure), and it further supplies the greenish-yellow stones frequently (though incorrectly) called "chrysolite" by jewelers. The material is very hard and reasonably tough and may be used in almost any suitable mounting.

SPINEL. After chrysoberyl come the materials rated as about 8 in hardness. First and hardest of these is spinel, then comes true or precious topaz. The various spinels are very hard and tough stones. The rough material persists in turbulent mountain streams where weaker minerals are ground to powder, and when cut and polished, spinel will wear well in any jewel. The author has long worn a ruby spinel in a ring on the right hand and has done many things that have subjected it to hard knocks, yet it is still intact, except for a spot that accidentally came in contact with a fast-flying carborundum wheel, which of course abraded the spinel.

TOPAZ. The true topaz is a bit softer than spinel, and the rough crystals show a very perfect basal cleavage. That is, they will cleave in a plane parallel to the bases of the usual orthorhombic crystals.

This being the case a cut topaz is very likely to be damaged by a blow or even by being dropped on a hard surface, and it would be wiser not to set such a stone in a ring unless it was to be but little used, or used by one who would not engage in rough work while wearing it. Thus a lady might wear a topaz ring on dress occasions for a long time without damaging it, but it would not do for a machinist to wear one in a ring.

GEMS BETWEEN 7 AND 8 IN HARDNESS. We now come to a rather long list of gem minerals ranging between 7 and 8 in hardness. Of these the princ.i.p.al ones are zircon, almandine garnet, and beryl (emerald and aquamarine) rated as 7-1/2 in hardness, and pyrope and hessonite garnet rated as 7-1/4 in hardness. Tourmaline and kunzite may also be included in this group as being on the average slightly above 7 in hardness.

The above minerals are all harder than quartz, and hence not subject to abrasion by the quartz dust which is everywhere present. In this respect they are suitable for fairly hard wear. The garnets are of sufficient toughness so that they may be freely used in rings--and the extensive use of thin slices of garnet to top doublets proves the suitability of the material for resisting wear. The zircon is rather more brittle and the artificially whitened zircons (known as jargoons) are especially subject to breakage when worn in rings. Fortunately jargoons are not commonly sold.

The beryl, whether emerald or aquamarine, is rather brittle. Emeralds are seldom found in river gravels. The material cannot persist in the mountain streams that bring down other and tougher minerals. The extreme beauty and value of the emerald has led to its use in the finest jewels, and the temptation is strong to set it in rings, especially in rings for ladies. If such rings are worn with the care that valuable jewels should receive they will probably last a long time without any more serious damage than the dulling of the sharp edges of the facets around the table. This slight damage can at any time be repaired by a light repolis.h.i.+ng of the affected facets. If an emerald is already badly shattered, or as it is called "mossy" in character, it will not be wise to set it in a ring, as a slight shock might complete its fracture. What has been said about emerald applies equally to aquamarine except that the value at stake is much less and the material is usually much freer from cracks.

Tourmalines, like emeralds, are brittle, and should be treated accordingly. Here, however, we are dealing with a much less expensive material than emerald, and if a customer desires a tourmaline in a ring mounting, while it will be best to suggest care in wearing it, the loss, in case of breakage, will usually be slight.

Kunzite, like all spodumene, has a p.r.o.nounced cleavage. It should therefore be used in brooches, pendants, and such jewels, rather than in rings. Lapidaries dislike to cut it under some conditions because of its fragility.

QUARTZ GEMS. Coming down to hardness 7 we have the various quartz gems and jade (variety jadeite). The princ.i.p.al quartz gems are, of course, amethyst and citrine quartz (the stone that is almost universally called topaz in the trade). As crystalline quartz is fairly tough and lacks any p.r.o.nounced cleavage, and as it is as hard as anything it is likely to meet with in use, it is a durable stone in rings or in other mountings.

In the course of time the sharp edges will wear dull from friction with objects carrying common dust, which is largely composed of powdered quartz itself, and which therefore gradually dulls a quartz gem. Old amethysts or "topazes" that have been long in use in rings show this dulling. There is, however, little danger of fracture with amethyst or "topaz" unless the blow is severe and then any stone might yield.

The many semi-precious stones which have a quartz basis (such as the varieties of waxy or cryptocrystalline chalcedony which is largely quartz in a very minutely crystalline condition) are often even tougher than the clear crystallized quartz. Carnelian, agate, quartz cat's-eye, jasper (containing earthy impurities), and those materials in which quartz has more or less completely replaced other substances, such as silicified crocidolite, petrified wood, chrysocolla quartz, etc., are all nearly as hard and quite as tough as quartz itself, and they make admirable stones for inexpensive rings of the arts and crafts type.

JADE. Jade, of the jadeite variety, which is rarer than the nephrite jade, and more highly regarded by the Chinese, is an exceedingly tough material. One can beat a chunk of the rough material with a hammer without making much impression upon it. It is also fairly hard, about as hard as quartz, and with the two properties of toughness and hardness it possesses excellent wearing qualities in any kind of mounting. True jade, whether jadeite or nephrite, deserves a larger use in inexpensive ornaments, as it may be had of very fine green color and it is inexpensive and durable.

SOFTER STONES. Coming next to those minerals whose hardness is 6 or over, but less than 7, we have to consider jade of the nephrite variety, demantoid garnet ("olivine" of the trade), peridot (or chrysolite, or the olivine of the mineralogist), turquoise, moonstone, and opal.

As has already been said of jadeite, the jade of the nephrite variety, while slightly less hard, is about as tough a mineral as one could expect to find. It can take care of itself in any situation.

The demantoid garnet (the "olivine" of the trade) is so beautiful and brilliant a stone that it is a pity that it is so lacking in hardness.

It will do very well for mounting in such jewels as scarf pins, lavallieres, etc., where but little hard wear is met with, but it cannot be recommended for hard ring use.

The peridot, too, is rather soft for ring use and will last much better in scarf pins or other mountings little subject to rubbing or to shocks.

Turquoise, although rather soft, is fairly tough, as its waxy l.u.s.ter might make one suppose, and in addition, being an opaque stone, slight dulling or scratching hardly lessens its beauty. It may therefore be used in ring mountings. However, it should be suggested that most turquoise is sufficiently porous to absorb grease, oil, or other liquids, and its color is frequently ruined thereby. Of course, such a change is far more likely to occur to a ring stone than to a turquoise mounted in some more protected situation.

The moonstone, being a variety of feldspar, has the p.r.o.nounced cleavage of that mineral and will not stand blows without exhibiting this property. Moonstones are therefore better suited to the less rude service in brooch mountings, etc., than to that of ring stones. However, being comparatively inexpensive, many moonstones, especially of the choicer bluish type, are set in ring mountings. The lack of hardness may be expected to dull their surfaces in time even though no shock starts a cleavage.

THE OPAL. There remains the opal, of hardness 6, to be considered. As is well known opal is a solidified jelly of siliceous composition, containing also combined water. It is not only soft but very brittle and it will crack very easily. Many opals crack in the paper in which they are sold, perhaps because of unequal expansion or contraction, due to heat or cold. In spite of this fragility, thousands of fine opals, and a host of commoner ones, are set in rings, where many of them subsequently come to a violent end, and all, sooner or later, become dulled and require repolis.h.i.+ng.

The great beauty of the opal, rivaling any mineral in its color-play, causes us to chance the risk of damage in order to mount it where its vivid hues may be advantageously viewed by the wearer as well as by others.

VERY SOFT STONES. Of stones softer than 6 we have but few and none of them is really fit for hard service. Lapis lazuli, 5-1/2 in hardness, has a beautiful blue color, frequently flecked with white or with bits of fool's gold. Its surface soon becomes dulled by hard wear.

Two more of the softer materials, malachite and azurite, remain to be described. These are both varieties of copper carbonate with combined water, the azurite having less water. Both take a good polish, but fail to retain it in use, being only of hardness 3-1/2 to 4.

LESSON XVIII

MINERAL SPECIES TO WHICH THE VARIOUS GEMS BELONG AND THE CHEMICAL COMPOSITION THEREOF

Although we have a very large number of different kinds of precious and semi-precious stones, to judge by the long list of names to be found in books on gems, yet all these stones can be rather simply cla.s.sified on the basis of their chemical composition, into one or another of a comparatively small number of mineral species. While jewelers seldom make use of a knowledge of the chemistry of the precious stones in identifying them, nevertheless such a knowledge is useful, both by way of information, and because it leads to a better and clearer understanding of the many similarities among stones whose color might lead one to regard them as dissimilar.

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.