Let us first consider what mineral species are most likely to give us green stones. Omitting the semi-precious opaque or translucent stones we have:
1. Grass-green beryl (the emerald) which is, of course, first in value among the green stones and first in the fine quality of its color.
2. Tourmaline (some specimens of which perhaps more nearly approach the emerald than any other green stones).
3. The demantoid garnet (sometimes called "olivine" in the trade).
4. True olivine (the peridot and the chrysolite of the trade).
5. Bluish green beryl (aquamarine).
6. Green sapphire (Oriental emerald or Oriental aquamarine).
7. Chrysoberyl (alexandrite and also the greenish yellow chrysoberyl).
I. Considering first the emerald, we have as legitimate a use of color in distinguishing a stone as could be selected, for emerald of fine grass-green color is not equaled by any other precious stone in the rich velvety character of its color. We have to beware here, however, of the fine glass imitations, which, while lacking the variety of true emerald, because of lack of dichroism, are nevertheless of a color so nearly like that of the emerald that no one should attempt to decide by color alone as to whether a stone is genuine or imitation emerald. If a hardness test shows that the material is a genuine hard stone and not a paste, then one who is well accustomed to the color of fine emerald can say at once whether a stone is a fine emerald or some other hard green stone. Where the color is less fine, however, one might well refuse to decide by the color, even when sure that the material is not glass, for some fine tourmalines approach some of the poorer emeralds in richness of color.
The "Scientific Emerald" Fraud. No "scientific" emeralds of marketable size have ever been produced as far as can be learned. Many attempts to reproduce emerald by melting beryl or emerald of inferior color have resulted only in the production of a beryl glass, which, while its color might be of desirable shade, was softer and lighter in weight than true emerald. It was also a true glass and hence singly refracting and without dichroism, whereas emerald is crystalline (not glassy or amorphous), is doubly refracting, and shows dichroism.
Do not be misled, then, into buying or selling an imitation of emerald under the terms "synthetic," "scientific," or "reconstructed," as such terms, when so used, are used to deceive one into thinking that the product offered bears the same relation to the true emerald that scientific rubies and sapphires bear to the natural stones. Such is not the case.
About the most dangerous imitation of the emerald that is ever seen in the trade is the triplet that has a top and a back made of true but pale beryl (the same mineral as emerald, but not of the right color) and a thin slice of deep emerald green glass laid between. This slice of glass is usually placed behind the girdle so that a file will not find any point of attack. The specific gravity of the triplet is practically that of emerald, its color is often very good, and it is doubly refracting. It is thus a dangerous imitation. (See Fig. 8.)
Emerald Triplets. A careful examination of one of these triplets, in the unset condition, with a good lens, will reveal the thin line of junction of the beryl with the glass. (The surface lusters of the two materials are enough different for the trained eye to detect the margin at once.) Such a triplet, if held in the sun, will reflect onto a card two images in pale or white light, one coming from the top surface of the table and the other from the top surface of the glass slice within. In other words, it acts in this respect like a doublet. A true emerald would give only one such reflection, which would come from the top surface of the table.
2. Tourmalines, when green, are usually darker than emeralds and of a more pronounced yellow green, or they may be of too bluish a green, as is the case with some of the finest of the green tourmalines from Maine. Connecticut green tourmaline tends more to the dark yellowish green, and Ceylon tourmaline to the olive green. The stronger dichroism of the tourmaline frequently reveals itself to the naked eye, and there is usually one direction or position in which the color of the stone is very inferior to its color in the opposite direction or position. Most tourmalines (except the very lightest shades) must be cut so that the table of the finished stone lies on the side of the crystal, as, when cut with the table lying across the crystal (perpendicular to the principal optical axis) the stones are much too dark to be pretty. Hence when one turns the cut stone so that he is looking in the direction which was originally up and down the crystal (the direction of single refraction and of no dichroism) he gets a glimpse of a less lovely color than is furnished by the stone in other positions. With a true emerald no such disparity in the color would appear. There might be a slight change of shade (as seen by the naked eye), but no trace of an ugly shade would appear.
By studying many tourmalines and a few emeralds one may acquire an eye for the differences of color that characterize the two stones, but it is still necessary to beware of the fine glass imitation and to use the file and also to look with a high-power glass for any rounding bubbles. The emerald will never have the latter. The glass imitation frequently does have them. The sharp jagged flaws and cracks that so often appear in emerald are likely to appear also in tourmaline as both are brittle materials. The glass imitations frequently have such flaws put into them either by pinching or by striking the material. Frequently, too, wisps of tiny air bubbles are left in the glass imitations in such fashion that unless one scrutinizes them carefully with a good lens they strongly resemble the flaws in natural emerald.
I have thus gone into detail as to how one may distinguish true emerald from tourmaline and from glass imitations because, on account of the high value of fine emerald and its infrequent occurrence, there is perhaps more need for the ability to discriminate between it and its imitations and substitutes than there is in almost any other case. Where values are high the temptation to devise and to sell imitations or substitutes is great and the need for skill in distinguishing between the real and the false is proportionally great.
3. The demantoid garnet (often unfortunately and incorrectly called "olivine" in the trade) is usually of an olive or pistachio shade. It may, however, approach a pale emerald. The refraction being single in this, as in all garnets, there is little variety to the color. The dispersion being very high, however, there is a strong tendency, in spite of the depth of the body color, for this stone to display "fire," that is, rainbow color effects. The luster, too, is diamond-like as the name "demantoid" signifies. With this account of the stone and a few chances to see the real demantoid garnet beside an emerald no one would be likely to mistake one for the other. The demantoid garnet is also very soft as compared with emerald (6 1/2 as against nearly 8).
4. True olivine (the peridot or the chrysolite of the trade) is of a fine leaf-green or bottle-green shade in the peridot. The chrysolite of the jeweler is usually of a yellower green. Frequently an olive-green shade is seen. The luster of olivine (whether of the peridot shade or not) is oily, and this may serve to distinguish it from tourmaline (which it may resemble in color). Its double refraction is very large also, so that the doubling of the edges of the rear facets may easily be seen through the table with a lens. The dichroism is feeble too, whereas that of tourmaline is strong. No one would be likely to confuse the stone with true emerald after studying what has preceded.
5. Bluish green beryl (aquamarine) is usually of a pale transparent green or blue green (almost a pure pale blue is also found).
Having all the properties of its more valuable variety, emerald, the pale beryl may, by the use of these properties, be distinguished from the pale blue-green topaz which so strongly resembles it in color.
6. Green sapphire seldom even approaches emerald in fineness of color. When it remotely suggests emerald it is called "Oriental" emerald to denote that it is a corundum gem. Most green sapphires are of too blue a green to resemble emerald. Some are really "Oriental" aquamarines. In some cases the green of the green sapphire is due to the presence, within the cut stone, of both blue and yellow portions, the light from which, being blended by its reflection within the stone, emerges as green as seen by the unaided eye, which cannot analyze colors. The dark sapphires of Australia are frequently green when cut in one direction and deep blue when cut in the opposite direction. The green, however, is seldom pleasing.
7. Chrysoberyl as usually seen is of a yellowish green. The fine gem chrysoberyls known as alexandrites, however, have a pleasing bluish green or deep olive green color by daylight and change in a most surprising fashion by artificial light under which they show raspberry red tints. This change, according to G. F. Herbert-Smith, is due principally to the fact that the balance in the spectrum of light transmitted by the stone is so delicate that when a light, rich in short wave lengths, falls upon it the blue green effect is evident, whereas when the light is rich in long wave lengths (red end of the spectrum), the whole stone appears red. The strong dichroism of the species also aids this contrast. The chrysoberyls of the cat's-eye type (of fibrous or tubular internal structure) are usually of olive green or brownish green shades.
Those who wish to further study color distinctions in green stones are recommended to see the chapters on beryl, peridot, corundum, tourmaline, chrysoberyl, and garnet (demantoid) in G. F. Herbert-Smith's Gem-Stones.
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Precious Stones Guide Vol 7
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