Transparent gem materials may be classified not only by the extent to which they reduce the velocity of light during transmission through them (i.e., refractive index), but by their optic character, In this assignment, one means by which broad categories of optic character may be differentiated from one another will be studied.
In order to identify gems effectively, often it is necessary to assemble all the information that the instruments at hand will yield. The polariscope is an inexpensive yet valuable instrument, therefore, it is important to make full use of its potentialities. An understanding of the optical theory presented in this assignment will make its reactions easy to interpret. The following discussions contain many references to light travel and the directions, or planes, in which it vibrates. To avoid any confusion between the terms vibration direction and direction of travel, refer to Figure A.
Amorphous materials and those crystallizing in the cubic system are singly refractive. Light is free to vibrate in any plane, and it travels at an equal velocity in all directions when passing through singly - refractive gems. This means that the light is not polarized. Singly-refractive materials are said to be ISOTROPIC.
Doubly refractive, or ANISOTROPIC, materials possess the property of polarizing light. A beam of light passing through them is confined to two sets of planes at right angles (polarised) except in optic axis directions, i.e., directions of single refraction in a doubly-refractive stone. The velocity of travel in the two planes differs, giving rise to double refraction. Gemstones crystallizing in the hexagonal and tetragonal systems are uniaxial. i.e., they are doubly refractive with one optic axis. Gemstones belonging to the orthorhombic, monoclinic and triclinic systems possess two optic axes and are denoted as biaxial.
Double refraction is an unusual and fascinating phenomena. It gives rise to the double images that are so helpful to the gemologist. The presence of C images can be explained by the polarization of light in doubly-refractive stones and the unequal velocity of the two rays resulting from polarization. When a beam of light strikes obliquely to the optic axis of a doubly-refractive stone, it is polarized into two rays at right angles which are being (refracted) unequally due to their differences in velocity.
When an object is viewed through a transparent, doubly-refractive material, a double image is seen. However, unless the material is thick, the object small or the double refraction strong, the object appears as two over-lapping images. Looking through the material, the side opposite to that through which observation is being made also appears to be doubled. For example, a given facet junction near the culet is seen as two parallel white lines when viewed through the table (refer to illustration in Assignment 2). Double image may be considered conclusive evidence of double refraction.
Doubly-refractive (anisotropic) gem materials can be classified on the basis of the number of optic axes and on optic sign. Whether a mineral is singly or doubly refractive and, if the latter, whether it is uniaxial or biaxial and positive or negative's called its OPTIC CHARACTER. Optic sign (positive or negative) a division of optic character, is defined-briefly in the following table, but a more complete discussion is given in later assignments.
One of the important advantages of determining the elements of optic character is that they are dependable characteristics of a gemstone. There are enough different categories to make the classification meaningful in identification.
The subdivisions of optic character will be useful in identification only if we can distinguish one from another. Fortunately, there are several methods that enable us to make broad categorizations. For example, it is usually simple to distinguish singly - from doubly- refractive gems, and often uniaxial from biaxial. Instruments used either primarily or incidentally to make such determinations include the polariscope, the polarizing microscope, the usual binocular microscope, the dichroscope and the refractometer. This assignment is concerned with the use of the polariscope for this purpose, before continuing with the discussion of polarization, the construction and use of the polariscope will be considered.
In gemology, a polariscope is usually used in preference to a polarizing microscope to analyze the optic character of gem materials. A typical polariscope is illustrated in Figure G. The housing acts as a partial light shield, as well as a support for the polarizer and the light source. The analyzer, which is the top Polaroid, is free to rotate. The polarizer, on the other hand, is fixed and is contained in a cell that also includes an unstrained cover glass and diffuser. The light source is located below the polarizer. Thus, light leaving the bulb and passing into the polarizer cell is first diffused by the diffusing element in the cell, this ensures an even distribution of ordinary light over the entire surface of the polarizer. It then passes through the polarizer, at which point its vibration direction is restricted to parallel planes of vibration. This polarized light may then be passed into a gemstone placed on the cover glass or held in the fingers between the polarizer and analyzer. Any modification of the polarized light due to the nature of the stone is observable through the analyzer.
Theory of Operation
Ordinarily, a beam of light is vibrating in all directions. perpendicular to its line of travel (Figure B3). A Polaroid acts as a selective filter, passing only that portion of the incident light which may be considered as vibrating in the plane of polarization (Figure B 3-2).
For a ray of light vibrating at right angles to the plane of polarization, no light is transmitted (Figure B 3-3). If the plane of polarization is rotated towards the vibration direction of the ray of light, the intensity of light transmitted will increase to a maximum when the plane of polarization is parallel to the vibration direction of the light(Figures B 3-4, B 3-5).
A ray of plane-polarized light (vibrating essentially in one direction, as after passing through a polaroid filter) entering a doubly refractive stone, is resolved into two rays polarized in directions mutually perpendicular (Figure B 3-6). If the light is not vibrating in a direction perpendicular to the analyzer, some of the light can pass and the stone appears light (Figure B 3-7). However, when the perpendicular vibration planes of the stone correspond to the perpendicular polarizer and analyzer transmission directions in the polarizer will be transmitted through the stone but this is then blocked by the analyzer so that the stone appears dark (Figure B 3-8). As the stone is turned and its vibration planes are no longer parallel and perpendicular to those of the polariscope, it will get lighter until it reaches a maximum brightness when the vibration planes of the stone are at an angle of 45° to the two polaroid vibration planes (Figure B 3-9). The result is that, as the stone is turned between two polaroid plates oriented at 90° to each other, it will appear to blink light o dark.
Although it is seldom encountered in practice, an ideal reaction for a singly-refractive transparent material would be to remain totally dark in the polariscope's dark position. Since such a material fails to polarize light transmitted through it, light polarized into a single direction by the polarizer reaches the analyzer without rotation thus, it absorbed by the analyzer and the stone appears dark.
Use of polariscope
In order to determine the optical characteristics of a gemstone in the polariscope, the analyzer is first turned to the dark position. The stone may be placed on the polarizer, as illustrated in Figure I or held either in a pair of tweezers or in the fingers between the polarizer and the analyzer. For most stones, the fingers are entirely satisfactory. If the stone is held between the thumb and forefinger, it may he turned and rotated readily for examination in various directions. First, rotate it in a plane parallel to the analyzer and polarizer. Then, after turning to another angle of observation with respect to the two plates, rotate it again parallel to them. This should be done several times, so that the stone is examined in at least three directions.
To save as much weight as possible, many uniaxial stones are cut with the girdle parallel to the optic axis (a direction of single refraction in a doubly-refractive gem), since the optic-axis direction is usually the elongated direction of the crystal. A deep tourmaline cut with a long direction parallel to the elongated direction of the original crystal will retain maximum weight. This general rule does not apply to the particularly valuable stones (ruby, sapphire and emerald), in which orientation to achieve the finest color demands that the optic axis be at right angles to the table.
An optic-axis direction is characterized in the polariscope's dark position by the presence of rainbow colors associated with an interference figure. The cause and analysis of interference figures by the polariscope is discussed in part B of the polariscope work later in the course. The observation of bright colors in a limited number of directions suggests optic axes and double refraction.
Thus, the interference colors characteristic of an optic-axis direction are usually seen in the polariscope through the center of the table of rubies and sapphires. In most other uniaxial stones, on the other hand, the table-culet direction is the one of maximum double refraction.
Biaxial stones seldom show any relation between the plane of the girdle and the optic axis.
The normal tendency for a person with little experience in the use of the polariscope is to drop any stone to be examined in the most convenient position (table down) and rotates it in that position. There is no particular objection to this, if several other positions are also used for the examination. The most serious danger concerns well-cut stones, particularly those that have a high R.I., such as diamond or zircon. They may-fail to transmit light, except through the culet, when placed table down in the polariscope. Such stones, which are cut to display maximum brilliancy, return all light entering the crown back through the crown facets; thus, it is returned to the polarizer. They can be analyzed correctly only when turned so that the light passes through them readily (Figure J). As illustrated, the easiest direction of analysis is approximately at right angles to the bezel facets. When a stone is placed on its pavilion facets over, the polarizer, little reflection is encountered, this is because the pavilion and bezel facets are roughly parallel, thus transmitting light readily. The table-down position is less likely to lead to analysis errors in stones of lower R.I., since more light can escape through the pavilion because of the larger critical angles.
If reflections cause difficulty in analysis, it is helpful to immerse the stone in a liquid. Water is usually satisfactory, but a liquid of higher R.I., such as bromoform (1.59) or methylene iodide (1.74) may be necessary, if the stone has a high index. When a liquid is used, most stones can be analyzed readily, even in a table-down position.
One other important factor is to make certain that a stone is sufficiently transparent so that enough light can pass through to be analyzed. Obviously, the polariscope serves no purpose in the analysis of opaque stones, moreover, its determinations are questionable when light is transmitted only through the thinnest edges of a stone.
HOW LIGHT IS AFFECTED BY DIFFERENT GEM MATERIALS.
If all stones to be tested in the polariscope were completely
transparent, unstrained, flawless, single crystals cut in spheres,
the polar scope test would offer no problems. However, most gemstones are flawed, many are twinned, some are aggregates of tiny crystals, the majority are faceted, and frequently they are mounted. Frosted or poorly polished surfaces may also affect results. A brief analysis of the manner in which light behavior is affected by these conditions is essential to an understanding of the common polariscope reactions.
Remember that light is not polarized in singly-refractive crystals, it may pass while vibrating in any plane. There are an infinite number of vibration planes at right angles to any given direction of transmission. Within these materials, light is usually refracted or reflected in a normal manner, but not polarized. An exception occurs when stones are under some degree of internal strain.
Strain may be caused by a number of factors. If any significant number of the positions in the space lattice are occupied by atoms of one or more other elements with different diameters, strain results. Strain may also result from solid inclusions, voids in the lattice, or other causes of lattice distortion. The distortion may give rise to partial polarization, usually of a spotty nature.
The effect of light passing through a strained singly-refractive material is illustrated in Figure C; this suggests the partial polarization of light in certain areas as it passes through the specimen. This means that, when tested in the polariscope, some of the light coming through the polarizer may be subjected to some rotation of polarization by the strained material, with the result that some of it passes through the analyzer. The strained stone thus appears to change in an irregular manner from dark to at least partial light transmission as it is rotated; this is known as strain or anomalous, double refraction.
Anomalous double refraction appears in a number of different patterns. The pattern may be irregular black streaks, a weak to strong black cross in a light background, or rarely an even extinction similar to true double refraction. The distribution of light and dark is controlled by the position of the areas in which strain has set up a sufficient degree of polarization to cause some light to pass through the polariscope. Irregular black streaks, which are perhaps the most common. Polarization caused by reflection from flat faces, or sides, of individual grains in crystalline aggregates or from inclusions in single crystals may cause some light to be transmitted by the analyzer.
If exceptionally strong, internal strain may crease a structural deformation that induces a sufficient amount of polarization and resultant double refraction to create interference colors. Certain amber and plastics may display such colors it tile polariscope. Strong strain may cause the same reaction in diamond. To a degree, in strained areas, the light is confined to two planes of vibration at right angles to one another, and the speed of travel permitted in each plane differs slightly. The light polarized into two planes with unequal velocities is put back into the same plane of vibration when it passes through the analyzer. Since the velocity difference within the gem has slowed one port on just slightly behind the another, they are slightly out of phase and interfere with one another therefore, when they are put back into the same plane of vibration by the analyzer, colors result.
The fact that such polarization encountered in amorphous materials is not equally distributed or oriented throughout the specimen results in an irregular display of interference colors. Similarly, they are present regardless of the direction through which the specimen is observed in the polariscope, in contract to truly doubly-refractive gems, which show colors only in optic-axis directions. Usually, strain colors are accompanied by grays and browns.
The simpler class of doubly-refractive single crystals is that
called uniaxial. They are so called because they have one optic
axis, or direction of single refraction. The planes of vibration
consist of planes oriented as shown in Figure D. Vertical planes
converge in the center, so that from the top an infinite number of
vibration planes exist about a given point. Through any given
direction from the side, however,
the orientation of the two vibration planes is at right angles, and all light passing through the specimen is polarized in these two planes (Figure E). The fact that light vibrating a these two planes travels at different speeds and is often subject to differences in selective absorption is explained by the different atomic arrangement, or spacing, between rows of atoms in these planes. The polarization in two sets of planes at right angles to one another means that light polarized by passing through the polarizer is rotated in the gem and passes through the analyzer (except when the planes of polarization of the gem correspond with those of the Polaroid plates). Thus, upon rotation, the gem passes alternately from light to dark with the dark position appearing every 90° during rotation.
Doubly-refractive single crystals that are biaxial have two optic axes, and the angle between these directions of single refraction varies among different gems. The vibration planes of such specimens are illustrated schematically in Figure F. Obviously, the analysis of this kind of a crystal in the polariscope is more complicated than for uniaxial stones, because of the more complex arrangement of the vibration planes. However, biaxial also alternately appear light and dark upon rotation, showing the dark position every 90° of rotation when examined in other than an optic axis direction.
Each of the multitude of minute crystals in a crystalline or cryptocrystalline aggregate of a doubly-refractive gemstone polarizes transmitted light. Since they are randomly oriented, emerging light vibrates in many directions. Also, since there is always some light which is oriented properly for transmission through the analyzer, aggregate always remains light as it is rotated.
CONFIRMATION OF SINGLE OR DOUBLE REFRACTION.
It is clear from the above discussion that the ideal reaction for a singly-refractive stone is to stay dark when rotated in the polariscope's dark position, for the doubly-refractive stone, darkening very 90° is the classic reaction. It is equally clear that the singly refractive stone seldom reacts in the classic manner. In the following paragraphs, methods of checking results are given.
There is one simple method for distinguishing between true and
anomalous double refraction. First, turn the polariscope to the dark
position (Figure K A). Then rotate the stone until it passes the
maximum amount of light (Figure K 3). Next, turn the analyzer Instrument's light position while the stone. If the amount of light transmitted by the stone increases as the analyzer is turned toward its light position, the stone is singly refractive. If it passes the same amount of light or darkens slightly, double refraction is indicated. If there is any doubt whether the stone passes more or less light, place an opaque shield of dark plastic or black paper with a hole cut in it either over or under the stone in the polariscope; this will eliminate enough of the background to determine whether the stone itself becomes lighter or darker. The shield should not be so large that it covers all the polarizer, because portion of the polarizer must be visible to determine when the instrument is in its light and dark positions.
If the stone is sufficiently transparent to pass enough light to analyze, singly-refractive behavioral be accepted without question. A doubly-refractive reaction is to be regarded with more caution. Occasionally, red garnets may be so strained as to give a doubly-refractive reaction. In this event, a dichroscope may be used to distinguish between garnet and doubly-refractive red stones, since ruby, synthetic ruby and alexandrite are strongly pleochroic. Garnet shows no dichroism. Other important transparent gemstones usually give results as expected. Diamonds are usually strained to some degree but they can be classified accurately, if examined in direction that permit's light passage.
REACTIONS AND INTERPRETATIONS.
Let us now consider the reactions that may be seen in the polariscope and the interpretations that may be given them:
A transparent stone rotated in the dark position of the
polariscope remains dark in all positions.
Interpretation: This is the classic reaction for a singly-refractive material. Few singly-refractive materials remain totally dark but may pass too little light in the dark position to cause difficulty, even to the novice.
- Reaction: The. stone shows a heavy black cross
in the center but no interference colors.
Interpretation: This is a typical reaction of glass or some other strained, singly-refractive transparent material, and should be confirmed by the method outlined in Section D of this assignment.
- Reaction: Extinction is
irregular and may be described best as patchy or cross hatched
(resembling in patches a coarse linen texture or irregular,
fragmented brushes). Usually, such a stone is never completely
light, but shows extinction in some portion during all or most
of the rotation.
Interpretation: Another type of strain-caused anomalous double refraction. It, too, should be confirmed by the test given Section D.
- Reaction: Patchy interference colors, together with greys end blacks, visible from any direction from within the stone. Interpretation: This description differs from the interference colors of a doubly-refractive stone, which are vivid and seen only parallel to an optic axis. The visibility in any direction, the patchy distribution, and the accompaniment of the greys and blacks indicate strongly strained, singly-refractive material, such as amber or other natural resin or a plastic.
- Reaction: Vivid interference colors are seen in one direction at 180°; or, in some cases, in two sets of directions at less than 180°, but not in all positions of t the stone. Rotation of the stone usually shows brushes. Interpretation: High Magnification should resolve the interference figure. In any event, the vivid colors visible in a limited number of directions offer proof of double refraction.
The stone becomes alternately light and dark on rotation, with a
dark brush, or band, moving across periodically.
Interpretation: This is a strong indication of true double refraction. The confirmatory test described in Section D should be applied, if, upon rotation along the brush, vivid interference colors are not present after turning the stone to sharpen the brush. This technique will be described in the advanced polariscope assignment.
The stone seems to change abruptly from light to dark on
Interpretation: This indicates either a doubly-refractive material at a considerable distance in angle from an optic axis or a strongly strained singly-refractive material. Such a stone usually can be identified easily by the confirmatory test already referred to.
The stone remains light in all positions in the polariscope's
Interpretation: This is the typical reaction of a doubly-refractive crystalline or cryptocrystalline aggregate. Strained translucent singly-refractive materials rarely show this reaction, but it is sometimes seen in the green jadelike grossularite garnet. Rotating the analyzer to the light position usually results in such a stone becoming lighter, as it does with transparent singly-refractive stones, or it may show a slight change of color. Doubly-refractive aggregates always remain the same or appear to darken. It should be noted that highly flawed stone as well as those with frosted backs may so diffuse the light passing through them that they will also appear light, regardless of position. This will not become a problem, if it is remembered that the reactions of such stones in the polariscope will always be questionable.
CARE AND MAINTENANCE OF POLARISCOPE
The polariscope requires no particular maintenance or adjustment. The main requirement for proper operation is that the polarize and analyzer be kept clean. Since the analyzer is unprotected film should not be touched with the fingers. If it does become soiled, it should be cleaned carefully with a moist tissue.
The bulb contained in the base is a standard 15-watt candelabra type, and should give good service for several years of operation. To replace it rarely requires removing the rubber feet from the instrument by unscrewing them, which releases the reflector plate to which the bulb is attached.
No attempt has been made to describe other styles of polar scopes, since, although they are different in design, the principle is basically the same.
The theory and testing techniques presented in this assignment emphasize the wealth of information that is available through polariscope determinations. The basic reactions studied above were limited to the determination of single and double refraction for a stone rotated between crossed polarizers. The determination of optic character and optic sign is the subject for this assignment.
Transparent doubly refractive materials can be analyzed in the polariscope to determine whether they are uniaxial or biaxial and positive or a negative in sign. In addition, the angle between the optic axes of biaxial stones may be approximated closely. Although some of these tests have only rare application in gem identification, the procedures should be understood in the event that difficult stones are encountered.
To determine whether a stone is uniaxial or biaxial, it should be placed in the polariscope with the polariscope in the dark position. The stone is then rotated slowly and observed in every direction until interference colors are obtained. The interference colors indicate an optic axis direction.
The interference colors may be located most easily by carefully watching the blinking pattern of the stone. When the stone is rotated in a position that is 90° to the optic axis, the entire stone will darken at once. At any other position it will not darken at once, but a dark shadow will move across the stone as the stone is turned. This dark shadow, or brush, will be narrower at one end than the other (Figure a). The narrow end points in the direction of the optic axis. By turning the stone in the direction that keeps the brush in view and in which the brush becomes progressively sharper (following the brush), the interference colors will generally come into view. Not all stones will show colors. The intensity of the colors varies. The intensity tends to be greater in uniaxial stones than in biaxial stones, in stones of lower R.I. than higher, and in smaller stones than in larger. The detection of interference figures in stones that display weak colors may require magnification up to 10x. A loupe or other magnifying lens may he used.
If the transparent doubly-refractive stone being tested in a sphere, it will not only show interference colors, but also an interference figure. The shape of the figure depends on whether the stone is uniaxial or biaxial. A uniaxial figure consists of a series of concentric colored rings intersected by crossed brushes (Figure b).
In some cases, the angle between the optic axes can be approximated or measured. This information is not often necessary to the gemologist, but it should be understood. A sphere or spherical cabochon of a biaxial material will usually show both optic axes easily. Four dots of ink placed on the stone to mark the points at which the figures are visible enable the observer to estimate the angle between the lines (representing the axes) that connect the dots. Faceted stones cause more problems since the reflection and refraction due to the facets cause distortion. However, the closer the two axes, the more the single brush will appear to bend. Thus, if the angle between the axes is 90°, the farthest apart that they can be, the brush will appear straight, but as the angle between the axes approaches 0°, the brush will appear more curved (Figure N). This ties in with the assignment on determining optic character with the refractometer. When the angle between the optic axes is 90°, beta will be halfway between alpha and gamma. In other words, the stone will be without sign. If the angle between the optic axes is close to 0°, the stone will be either strongly positive or strongly negative.
Brushes and interference colors are caused by a combination of the behavior of light passing through a stone along the general direction of the optic axis and the effect of the polariscope on this Figure O and P shows the behavior of light as the stone is viewed parallel to the optic axis. The central line of Figure O and the center of Figure P representing different views of the optic axis direction, show the light vibrating in all directions since this is the direction of single refraction. However, as the light moves through the stone in a direction other than the optic axis direction, it is split into two beams vibrating at right angles to each other (Figure O). The arrows in Figure B represent these perpendicular vibration directions and show how they are arranged radially about the optic axis. When a stone is placed in this position in the polariscope's dark position, the light that is vibrating perpendicular to the polarizer is blocked and cannot pass, just as described in Assignment No. 6. (See Figure Q). Because this light is then vibrating in a direction perpendicular to the analyzer, it cannot pass and this section of the stone appears dark. The light vibrating in the other directions passes and the combination of the light and dark areas causes the crossed brushes of the uniaxial figure (Figure R). The biaxial figure is caused by essentially the same process but it is more involved due to the presence of two optic axes.
Interference of light is caused by beams of light overlapping and alternately cancelling and intensifying (interfering with) each other. The light entering a doubly refractive stone is split into two beams travelling at different speeds in the same direction and vibrating at right angles to each other. The beams are overlapping but they must be vibrating in the same direction for interference to take place. However, after the beams haven passed through the analyzer, they are all vibrating in the same direction, interference takes place and the interference colors result.
A misconception that many people have about an optic axis direction is that they visualize this direction much as they would picture a single needle pushed through a block of wood, this is not the case. The optic axis may be observed through a stone at all points of the stone in a parallel direction.
Optic sign (positive or negative) determinations can be made with the polariscope, but this test is used primarily by mineralogists rather than gemologists. To make this test it is necessary to have a quartz wedge, an accessory to a polarizing microscope (Figure T). It is a thin wedge of quartz that has been pre-oriented for this kind of work and cemented to a glass cover plate. The entire unit is usually mounted in a metal holder designed to fit into the tube of a polarizing microscope. When used with the polariscope, it is held in the hand. This makes it very awkward to use with the polariscope since the stone and possibly a glass ball and magnifier also need to be held and manipulated at the same time. This test is rarely required and then only when separating certain unusual gem species.
Once an interference figure is resolved, the wedge is slowly inserted in the field of view just over the stone. The behavior of the colored concentric rings through which the wedge is passed is noted in relation to the vibration direction of the quartz wedge which is marked by an arrow stamped on the metal holder. For uniaxial figures, the wedge is inserted through the center of the figure and between the brushes (Figure U). As it is moved in slowly, the colored rings in one set of opposite quadrants appear to move out from the center, while those in the other quadrants move in. If the movement is away from the center at right angles to the arrow, the stone is positive, if toward the center, negative. In biaxial stones, the wedge is passed over the figure entering from the concave side of the brush (Figure ). The interpretation of the movement of the colored rings is the same as for uniaxial stones.
The polariscope may be used to determine single or double refraction as well as aggregate reactions. It can also be used to determine whether a doubly refractive stone is uniaxial or biaxial or positive or negative in sign. It is perhaps the most difficult instrument for the novice to master.