Introduction
Examination With Unaided Eye was devoted to the visual examination of gemstones by the unaided eye and the loupe. Instructions were given on the use of the loupe, proper lighting for maximum efficiency, and what to look for with the loupe. This assignment is devoted to the use of higher magnification in the form of the microscope, with a further discussion of those characteristics that are detectable by the effective use of magnifiers of higher power. Attention will be given to binocular magnifiers with lighting systems designed for maximum effectiveness with gemstones.
The usual monocular microscope reverses the image of the object viewed, which makes it difficult to use. A stereoscopic binocular microscope equipped with wide-field eyepieces not only gives erect images, but the twin eyepieces and objectives permit stereoscopic observation, thus providing a depth perception that is impossible with a single magnifier.
Basic Facts Regarding Magnification and Magnifiers.
A lens that increases the apparent size of an object seen through it is known as a magnifier. A single lens may accomplish this if it is narrower at the edges than at the center, or it may be accomplished by the use of a series of individual components. If a lens is thicker at the center than at the edge, it magnifies and is said to be positive. If it is thinner at the center than at the edges, it reduces the apparent size of the object and is said to be negative.
The size of the magnifier also is important, generally, in order for a simple lens to have a wide field and an effective depth of field and working distance, magnification must be sacrificed. The higher the magnification and the shorter the working distance, the greater the problems of lighting the object adequately. Although the factors of working distance and depth and size of field are important in observing gemstones, in most identifications they must be sacrificed to some extent for considerably higher magnification. This may be overcome somewhat by using combinations of lenses that achieve both width of field and an effective working distance.
The wide variety of uses to which an effective magnifier can be put, coupled with the fact that it is essential for the detection of synthetics and imitations, makes it the most important tool available for gem testing. In 1938, Robert Shipley, Jr., of the GIA, designed and constructed an illuminator in which the adapted for gem use the highly efficient dark field illumination. It was made in a manner that permitted rapid shifting to a form of light field illumination. The illuminator was made in the form of a base on which a binocular microscope was mounted. Many advances since that time in design and construction have resulted in today's highly effective equipment, incorporating such features as an iris diaphragm to give the correct amount of light and the new Gemolite's Stereozoom feature that permits a continuous change of magnification from ten to forty-five or ninety power. Both the Gemolite and the Diamondscope trademarked names for widely known instruments, mount binocular stereoscopic magnifiers on illuminator bases.
The magnification of a microscope is determined by the magnification of its objectives, the length of the tube or tubes, and the power of the eyepieces. Since the objectives and eyepieces of a standard compound microscope are designed for this particular kind of instrument, the total magnification is determined by multiplying the power with which the objective is marked by the power marked on the eyepieces. For example, a 1x objective with a 10x eyepiece give a 10 power magnification; a 4x objective with a 5x eyepiece gives a 20 power magnification. The new Stereozoom design changes the magnification of the system by utilizing elements between the objective and eyepiece lens to provide the equivalent of an increase in tube length. In effect, the eyepiece and objective remain constant but tube length changes. Tube length does not actually change, but it does in effect, because the movable elements in the body increase or decrease the distance the light travels between the objective and eyepieces.
Most monocular microscopes that utilize the usual eyepieces and low aperture objectives require very close working distances, and the width of the field is very restricted. Only a small portion of a gem, even of a one carat diamond, can be seen in the instrument at ten magnifications. On the other hand, a microscope equipped with wide-field eyepieces permits a considerably greater field; and by the use of lower powered, wide aperture objectives, together with a fairly high powered eyepiece, great working distances are possible. This makes for convenience in the examination of rings and other jewelry. For work with gemstones, magnifications of approximately ten to slightly less than one hundred are practical. The higher the magnification, the shallower is the depth of field, the smaller is the field, and the greater is the difficulty of lighting the object. For almost all purposes, magnifications of ten approximately forty-five are entirely satisfactory.
Because as objectives and eyepieces do not have to be paired and because it does not have prisms to provide erect images, a monocular microscope affords high magnification less expensively than a binocular instrument. However, in the opinion of many gemologists, a monocular microscope used purely for magnification has little value in gem testing.
It is so much more difficult and time consuming to use effectively that a person who values his time finds it highly impractical. Often identifying characteristics that can be located quickly with a binocular instrument and dark field illumination can be found only after prolonged effort by an equally capable man using a monocular, if indeed, he is able to locate them at all. To one who uses a monocular exclusively this may seem like an overstatement, but one who has access to both types will agree wholeheartedly. The difference in utility is so great that all but the lowest quality binocular magnifiers are much more valuable than the most expensive monoculars.
A binocular microscope designed for effective gem use is sufficiently costly that a gemologist may feel unable to afford its purchase for some time. If this situation exists, he has several choices. He can buy a monocular; an inexpensive binocular, such as the Gem Detector (Figure 2); a fine binocular without illuminator base, to which a base can be added later; or he can refuse any identification that cannot be made with a loupe. The least waste in any interim action is that in which only the quality binocular portion is purchased.
Instructions for using a Binocular Microscope.
Instruments such as the Gemolite, Diamondscope and Gem Detector were designed specifically for the examination of gems. The first two are used widely as aids in selling diamonds and grading them for clarity and cutting quality. The Gem Detector, with magnifications of 20x and 30x only, is intended solely for gem-identification purposes. All three magnifiers feature a spring loaded stone holder mounted in a manner that permits both steady positioning and a wide latitude of movement for both mounted and un-mounted stones. Most of the instructions given in Examination With Unaided Eye for cleaning and mounting a stone or jewelry piece for loupe examination apply equally to the binocular microscope. Usually, however, it is much less difficult to distinguish between surface objects and inclusions, and less time and trouble are required to locate the key identifying features.
After cleaning, the loose stone is placed table down, without touching it with the fingers, and the jaws of the mechanical stone holder are opened and closed gently onto the girdle. The holder is then pressed over the post on which it rests; this provides for several degrees of up-and-down motion and a full 360° rotation about the axis of the holder.
Internal Characteristics.
The first step is to examine the stone in a dark-field position; i.e., with the black background in place. First, note any internal breaks, by focusing from table to culet and examining slowly all portions of the stone in the process. Even before an attempt is made to identify the stone, its physical condition should be studied. If it belongs to a customer, attention should be called to any damage before it is accepted for whatever purpose he brought it to you. Note the extent and the potential danger to the life of the stone that is suggested by present breaks. Call this to the owner's attention, warning him especially if they seem likely to extend. If you are considering the stone for purchase, either from a dealer or from a layman, make sure that it can be handled without too much risk.
Still under dark-field illumination, examine any fractures or cleavages that extend to the surface for evidence of crack filling (probably with liquid) that has been done for one of three purposes (1) to conceal the fracture, by cutting down reflection from an air film in the opening; (2) to impart color to an otherwise too-pale stone; or (3) to bring about the formation, along the break, of natural-appearing crystals that grow as the saturated solution dries. These give the impression of angular inclusions in a manmade stone.
Rubies and emeralds are sometimes "oiled" so that flaws are less obvious. The R.I. of the oil, which may be colorless or have approximately the gem's color, is nearer to that of the stone than to the air that would otherwise fill the break. As a result, the oil often makes the break virtually invisible. Obviously, this should be guarded against in buying as well as in appraising. But the greatest danger to the retailer's reputation and pocketbook is to accept such a stone for repair or resizing and inadvertently remove the oil in a pickling solution or in a steam or ultrasonic cleaner. There is no doubt that the retailer would be accused of damaging the stone. Frequently, jewelers believe that they or an employee damaged a ruby or an emerald in some unaccountable fashion, when their only fault was failure to recognize the presence of oil.
An oiled stone may be difficult to detect. Usually, unfilled fractures in a ruby or an emerald appear mirror like when light is reflected from them. If they are difficult to see, even under magnification and when surface traces of the fractures are evident there is reason to suspect oiling. Usually, a loose stone of fair quality or better is placed in a paper with a cotton insert for protection. Thus oil usually is apparent from the color that has stained the cotton. Sometimes, it may be seen on the paper as well. Even a mounted stone may seen enough oil to soil its container.
Under magnification, the oiled fractures are more difficult to see, but they seem to have more color than the remainder of the stone. This is not to be confused with color banding. In other words, liquid fractures fail to cause mirror like reflections, but seem to have more; rather than less, color than the surrounding material. Emeralds and rubies that are set so that the pavilions are covered are particularly likely to have had their fractures filled, and in this situation the oil may be used to increase the stone's depth of color greatly. The gem material used for this type of setting often is much too pale to be of fine quality, so color is added and the fractures are hidden in the same operation.
To help determine whether color has been added by this method, place a piece of opal glass or white paper over the diaphragm under the stone, to see if the crack shows more color than the rest of the stone. Be sure you are not just getting an interference-color effect. If the color in the fracture does not change after turning the stone a few decrees, interference is not indicated. Later in this assignment, under the heading of surface characteristics, it will be explained how to determine whether the fractures or cleavages existed at the time the stone was cut.
The next step is to study any inclusions that may be present. If any doubt exists as to whether they are indicative of natural or manmade origin, check the appropriate assignments in the colored gemstone assignments. Inclusions characteristic of synthetic and other manmade stones are to be found also in the supplement with this course. The subject of the use of characteristic inclusions for identification is covered in detail in the assignment following the next work project. At this point, it is important to examine the pattern of inclusions under low power. Often, inclusions that appear to be unrelated individual entities will be seen to form a very revealing pattern when the stone is examined in this manner. For example, in ruby, sapphire and emerald, a number of inclusions or lines that together present a hexagonal aspect may assist materially in identification. The pattern of rutile needles oriented in three directions at 60° to one another in the same plane is a very strong indication of corundum, or at least of the hexagonal crystal system (Figure 3).
During the initial examination, remember the possibility of facet edge doubling that may be apparent on the side opposite that through which observation is being made. Scratches, lint or other objects on the opposite surface are likely to show doubled images also. Remember that many of the important gems, including sapphire, emerald, aquamarine, tourmaline and zircon, are uniaxial and that usually they are cut so that the optic axis is perpendicular to the table. As a result, when looking through the center of the table toward the culet, no doubling will be in evidence, even when the birefringence is as strong as that of zircon. For this reason, it is important to examine the stone through the crown facets near the girdle, in order to get an idea of the magnitude of birefringence, if any is visible. It usually is difficult for a novice tester to detect doubling at first. The tendency is to assume that the eyepieces are not properly aligned or that observation is being made through two different facets, so that the doubling effect is unconsciously dismissed.
In the work project that follows this assignment is included a series of stones with birefringence's ranging from almost 0.180 down to 0.005. Even those with a 0.009 value are large enough so that doubling should be visible under 20x, either by loupe or binocular microscope. With practice, an able gemologist can judge birefringence to +0.005 readily. For example, you should be able to separate amethyst and tourmaline easily on the strength of birefringence alone. Use stones of known identity, ranging from the 0.006 of aquamarine to the 0.008 of corundum, synthetic corundum and topaz through the 0.009 of quartz, the .018 -.020 of tourmaline, the 0.036 of peridot, to the 0.059 of blue or white zircon. The first step is to be sure you can always distinguish birefringence, and the second is to be able to measure its strength by eye accurately.
Another feature that should be revealed by magnification and proper lighting is unusual structural lines. These may be very straight discontinuities attributable to twinning, the curved accumulation lines associated with certain colors of synthetic corundum, or the irregular swirled effects seen in glass. Twinned effects are often seen in corundum, diamond, green zircon, and a few other gemstones. One or more twinning lines may be seen rarely in synthetic corundum.
Another interesting feature under magnification is that has some importance in the identification of natural stones has been referred to variously as a heat-wave or roiled effect. This effect, accompanied by irregular patches almost without color in brilliant-red Burma ruby, is characteristic. The roiled or oily appearance that is characteristic of hessonite garnet is illustrated in Supplement.
Color distribution is another feature that may be visible when a light background is produced by placing white paper or opal glass over the diaphragm. Curved or straight color bands often become evident when examined in this manner; the straight banding is characteristic of natural stones and the curved of synthetic. In addition, a spotty color distribution may be evident. Sometimes, the shape of the banding or spot gives a strong clue to the crystal system of the stone.
Usually, this examination of gemstones over a low intensity diffused-light background reveals doubles and triplets by the differences in color in their different parts.
If difficulty is encountered in resolving distributional irregularities of color, and this seems essential to a sure identification of natural or synthetic corundum, immerse the stone in methylene iodide over a low intensity light background. This provides a material increase in efficiency.
Surface Characteristics
In order to detect and study surface characteristics, it is wise to examine transparent stones under both dark-field and overhead illumination. For the latter, the diffused light of a fluorescent lamp is satisfactory.
First, under dark-field conditions, examine the girdle for evidence of cleavage and fracture and for the luster on broken surfaces. Often, the character of the girdle identifies diamonds quickly and surely. These is a new diamond substitute, GGG, that approximates the waxy smooth girdle surface of a carefully turned diamond. The presence of cleavage in a colorless gem of very high index strongly suggests diamond (although it is possible in some other materials rarely cut as gems), and one or more naturals (the shiny, original surface of a diamond crystal) on the unpolished girdle indicates a diamond. Triangular growth marks, if present on the natural, also indicate diamond.
Fractures or other breaks are most often revealed along the girdle of a stone. If these are definitely cleavages, the number of possibilities is reduced materially. The luster on fracture surfaces is of particular interest on nontransparent stones; on turquoise, chalcedony, coral, malachite, hematite, jade and many other translucent to opaque stones it is not vitreous but waxy to dull, and the shape of the break may be other than conchoidal. Nontransparent glass often may be detected by this test alone.
The nature of the surface often provides clues to the identity of a stone. On what appears to be a faceted gemstone, evidence of molding, particularly on back facets, is a sure sign of glass or plastic. A study under magnification shows that the shiny surface was not polished but formed in a mold. The facet's surface is often concave instead of flat and is characterized by an appearance similar to that of an organge peel. A polished gem will never show a concave facet.
Polishing lines can be seen on almost any stone, even if it is well polished. However, in order to see them, it may be necessary to use fairly high magnification and illumination that is almost parallel to the surface of the facet. It a stone has been polished very rapidly (a characteristic of stones of little value, such as synthetic corundum and spinel), tiny fractures at the surface resulting from high temperatures generated in the rapid polishing are often evident.
A study of polishing characteristics is used by trained gemologists to determine whether fractures or cleavages existed at the time the stone was polished. Polishing lines, or grooves, are common to almost all polished surfaces, although high magnification is sometimes required to resolve them. Irregularities in the polishing wheel, as well as variations in particle size of the abrasive, may cause them. Such lines will extend across an entire facet. In addition, shorter grooves, referred to as drag lines, may be encountered that extend from pits or fractures at the surface. Those are caused when tiny particles of abrasive or fragments of the stone that are chipped or dislodged from the sharp edge of a break are forced into the surface leading away from the break, causing it to be scored.
Very high magnification and parallel lighting are used to study the polishing lines on both sides of fractures or cleavages that reach the surface. Lines or grooves will be seen to continue on both sides of the break, if it did not exist at the time the stone was polished. If it did exist, grooves on one side will not match those on the other, and there will be a tendency for one lip to be sharp and the other to be slightly curved. The polishing wheel tends to minutely break away the first edge and to slightly curve the opposite side of the break. This often is of significance in insurance problems, for it enables the gemologist to distinguish between breaks that existed when the stone was polished and those that occurred later. The presumption is that the damage occurred while the stone was the property of the customer, if they postdate the polishing.
Sometimes, concussion marks are visible near major breaks. These may take the form of minute pits or they may be round or nearly equidimensional, geometrically formed minute indentations with a tendency for the edges to be turned up.
The nature of facet edges often furnishes clues to the identity of a stone. The polishing wheel tends to round the facet edges of stones that are less than 7 in hardness, such as strontium titanate and synthetic rutile. There also is a tendency for facet edges to be abraded, particularly if a stone is 8 or under in hardness and has been worn for a year or two may show concussion marks and tiny nicks on facet edges.
The tester should be alert to the possibility of surface coloration. For example, a rather resistant material such as magnesium fluoride is frequently sputtered onto the backs of zircons in a near vacuum. If the entire pavilion is treated in this manner, an iridescence will be detectable under fairly high magnification. A coating used on diamonds in recent years is confined to the girdle and near-girdle pavilion area (Figure 5). Iridescence, however, rarely marks its presence; in fact it is sometimes necessary to study a stone at great length under very high magnification before any evidence of the coating can be seen. If the girdle is largely concealed by the mounting, detection may be impossible. When detected, it usually appears as a very faint purple line just below the girdle. Often, it is easier to detect a coating by looking through the stone than by observing the surface. Color filters may increase the contrast between coated and uncoated areas, and thus the visibility of the coating.
Some emeralds have a thin coating that is natural rather than artificial. The cause of the coating, which superficially resembles tarnish, is not known. It does not affect the unaided eye appearance of the emerald or detract from its value, but it could be confused easily with an intentional coating.
Glass often tarnishes over a period of time to such an extent that it must be polished lightly before an R.I. reading can be obtained. The tarnish appears faintly iridescent under magnification.
