For forty years after the manufacture of the first refractometer specifically for gem-identification purposes, it was used only to obtain readings on stones with large, flat, well-polished surfaces. In 1948 a testing method that increased the usefulness of the instrument enormously. The SPOT METHOD, as it is called, permits indices to be obtained within the 1.40-1.81 range on any polished flat or convex surface, if the correct technique is used.
The spot method requires much less magnification of the contact between stone and hemisphere. The British call it the "distant-vision method" because the scale is viewed from a distance of about five to eighteen inches, depending on the hemisphere and the magnification provided by the lens elements. In the original method of refractometer use, a facet had to be rather large to provide enough area for the contrast between light and dark to be clearly visible through the eyepiece. The best readings were, and are, seen when the facet covered the full width of the scale. With instruments of high magnification (e.g., the Rayner), to the point of the reading. The Gem Refraxtometer, with its rather low magnification (unless the eyepiece is in place), does not do this, and the stone with a large flat facet usually appears as a large spot on the scale, from which is reflected the image of the contact between the facet and hemisphere. When very small stones or curved surfaces are tested, there is only a small spot of contact. Assuming that only a small amount of liquid is used to provide an optical contact, the shape of the stone's surface will determine the shape of the spot. In Figure A for example, the resutant spots for three stones are illustrated; a large table facet, a cabochon, and a small stone with a triangular table. If the true nature of the stone's surface is not apparent, in all probability too much liquid has been applied; this may cause difficulty, particularly with cabochons or tiny faceted stones. With small contact areas, the amount of light reflected is not sufficient to produce a visible reading by a lens system designed to magnify the full width of the scale. Therefore, it is necessary to see beyond the scale and actually observe, under very little magnification, the change that occurs when the critical angle (or angles) of the contact area is passed as the eye is moved up and down the scale; i.e., scans the are of the hemisphere.
The eyepiece of the Refractometer is fixed in position but it is of low power, so that the scale and contact area are both visible with the eye only a few inches away. Much of the needed magnification is supplied by the huge hemisphere (actually, a section of a hemicylinder). The effect produced by moving the eye up and down the scale is accomplished by moving the mirror within the instrument.
SPOT-READING TECHNIQUES
The first step in the determination of refractive index by the spot method is to place on the hemisphere as small a drop of liquid as possible. Looking into the instrument from a distance of a few inches the liquid will be seen as a dark spot that moves in relation to the scale as the eye is moved. The great distance between eye and eyepiece multiplies the difficulties of becoming accustomed to this method on the Rayner. Often, it takes a long time before the tester can learn to find the spot readily. More detailed directions for Rayner use are given later in the assignment. If the liquid being used has an R.I. of 1.81, it will be visible from the low numerical end of the scale to the 1.81 line, at Which point it will disappear. When a stone with an index between 1.40 and 1.80 is placed on the spot, at some point on the scale the appearance of the spot will change.
To understand the underlying theory of the spot reading, place a small stone with a flat facet on the hemisphere and take a normal reading with the eye close to the eyepiece. Now make the same observation from a distance of several inches. By effectively reducing magnification, the contact between stone and hemisphere will be seen as a spot. It will be noted that the spot remains dark at the low-number end of the scale up to the point at which the reading was observed with the eye close to the eyepiece. At that point the center of the spot changes from dark to light (probably separated by the usual spectrum that marks a white-light reading). Only the ring of excess liquid around the contact remains visible up to the 1.81 position. The same holds true when a cabochon surface is in contact with the hemisphere, except that at the point of the reading the curved surface causes the dark and light sides of the circle or oval-shaped contact to be reversed. In other words, at the point of the reading, the light side of the circle or oval toward the low-number end of the scale and the dark side is toward the high-number end. The reading however, is still the division between the light and dark sides (Figure B).
As pointed out above, the amount of liquid used must be kept to an absolute minimum. Most testers use too much on regular flat readings; in fact, stones that have low S.G.'s sometimes actually float on the liquid. Moreover, excess liquid is likely to damage the refractometer by working down through the seal around the hemisphere, where the sulphur crystallizes and masks readings. The drop should be so small that the resultant spot of contact between the curved surface and the hemisphere covers no more than two scale divisions. When excess liquid is used, it is not uncommon for the spot to cover more than ten divisions; e.g., from 1.6 to 1.7.
The best technique for liquid application is to first touch the applicator rod repeatedly against the mouth of the bottle, to remove as much of the liquid as possible, and transfer a tiny drop to the surface of the hemisphere (Figure C). Avoid tapping the soft glass of the hemisphere with the rod. Then touch the stone to the drop of liquid, and observe the resultant spot. If it is more than two divisions of the scale in width, lift it so that the hemisphere can be wiped off. A small amount will remain on the stone. The stone is then replaced and the resultant spot produced by the liquid on the stone is observed again. If still too large, pick up the stone and again remove that portion of the liquid that remains on the hemisphere. Repeat this until the drop is reduced in size to one or two scale divisions. When the spot is this small, it is easy to pinpoint the number at which it changes from light to dark as the eye is moved up and down on the scale. If, for example the spot is five divisions wide, it is often difficult to determine exactly at what point the change occurs. It is necessary to reduce the amount of liquid, as explained, since the only area of the stone that will provide an accurate reading is the one that is essentially parallel to the hemisphere surface (Figure D). If part of the curved surface is exposed to excess liquid, light will be scattered from the curved edges and the reading will become diffused (Figure E). If the surface being tested is a tiny facet, it is necessary to reduce the liquid to an amount sufficient to reveal the shape of the facet.
Unless great care is used, the spot-reading technique is likely to hasten the abrasion of the hemisphere. Just the repeated placement of the stone as the liquid is wiped off increases wear. Moreover, the area of contact is much smaller than that between a large, flat facet and the hemisphere surface. Not only is there a concentration of weight but the cabochon often must be hand held, thus causing pressure to be exerted on a single point of contact. Although this method makes it possible to test a large objects, such as carvings, it must be done very gently; otherwise, the relatively great weight bearing on a point of contact may cause severe damage. Even when utmost care is exercised, hemisphere damage is frequent. Thus, the instrument should be used for this purpose only when other tests fail to produce the necessary information. Usually, a cabochon is not nearly as troublesome, because it is seldom as heavy and is less difficult to handle. However, more difficulty is encountered when it will not rest on the hemisphere's surface without being held in the hand. If this is necessary, a compulsive movement of the fingers may cause scratching, even if the stone is flat. Thus, the use of the spot method enquires much more caution than an ordinary flat-facet reading.
When taking a reading on a tiny facet, only an amount of liquid sufficient to cover the facet should be used. The shape of the facet should be outlined in the spot, without a wide, dark collar of liquid. Adjusting the amount of the liquid is accomplished in the same manner as with a cabochon.
A number of other points should be kept in mind when making spot readings. When white light is used (the only practical light source for this purpose), the position of the reading may be indicated by a blue-green line at the division point between light and dark, as it is in a regular reading. Usually, this happens only when the liquid spot for a cabochon is larger than necessary or when a small, flat facet is in contact with the hemisphere. With a spot reading of the right size, the entire spot may become blue; this, of course, indicates the position of the reading. A sharp reading may be taken if the spot is tiny and changes abruptly from light to dark or if there is a point at which it is half light and half dark. If it changes over several divisions, the index is the average between a reading at a point where the spot is last all dark and a second reading where the spot is first completely light as the eye mobes from the low to the high number end of the scale. For example, if the highest numerical point at which the spot is all dark is 1.52 and the first at which it is all light is 1.58, an average would indicate a reading of about 1.55 (refer to Figure FA).
If a stone is poorly polished or has a slightly irregular surface, sometimes the spot changes slowly over a rather large distance on the scale. (This can happen also if too much liquid is used). This may occur through a range of even .1 on the scale. In this situation, the first step is to attempt to sharpen the division by removing considerably more liquid. If this does not help, the same procedure recommended for a smaller spot should be used. In this case, of course, the reading must be considered accurate only to plus or minus several scale divisions.
This is well illustrated in Figure FB. If the two figures were 1.50 and 1.66, the 1.58 average would be safe only to ± .02 or .03. The index would be assumed to be between 1.55 and 1.61.
If the curved or flat surface being tested is elongated, the spot
should be turned so that the long direction is parallel to the
length of the hemisphere; this makes it much easier to determine the
point at
which the spot shows light and dark divided. If a stone
has a very slightly curved surface, it is necessary to use the spot
method rather than to attempt a flat-facet reading with enough
liquid to cover the facet; otherwise, depending on the angle of
observation, a wide range of readings may be obtained. If the amount
of liquid is greatly limited and the spot method is utilized, a
sharp reading is possible.
The spot technique with the Rayner, which has both a fixed eyepiece and high magnification, is considerably more complicated than with other instruments, because it is necessary to withdraw the eye to a distance of at least twelve inches before the spot becomes visible. The spot may not be easy to see at first, because it is difficult to bring it into focus at the same time as the scale. It is important that the stone be placed exactly at the center of the hemisphere's flat top surface. To experiment, a stone with a fairly small flat facet should be used, so that it can be checked by a close-up reading. After placing the stone, withdraw the eye slowly to a distance of about fifteen inches while looking for the spot. When it finally becomes visible, probably it will be impossible to see the scale. Then it is necessary to bring the eye close to the eyepiece and again move away slowly, keeping a portion of the scale in focus until the spot also comes into view. Since only a portion of the scale can be seen, holding the spot in focus while trying to determine the scale reading of a change from dart to light is difficult at first. It may be necessary to repeat the operation several times before both can be seen simultaneously at the point where a reading can be taken.
Even a small drop of liquid on the Rayner appears huge at a distance of twelve inches or more from the scale. The strength of the magnification makes the scale too large for more than a very small portion of it to be visible at any one time at this distance from the eyepiece. It is necessary to view the scale at a slight angle to one side of center, in order for the number to be still visible at a twelve to fifteen inch distance. Usually, it is best to choose one of the numbers on the scale, such as 1.5 or 1.6, to use as a guide mark as the eye is withdrawn: by the time this distance is reached, the number will appear to be very large (Figure G). Although the high magnification adds greatly to the difficulty in-obtaining a reading, the fact that the scale divisions are magnified sometimes makes it possible to get a particularly satisfactory reading. It is a very difficult instrument to use with a large object that has to be held in the hands for a test to be made.
On the Rayner, as well as on other instruments, it is very helpful to practice first with stones that have flat facets large enough to be checked by the usual method of holding the eye next to the eyepiece. After becoming adept at reading such stones, it is advisable to practice with cabochons that have polished flat backs; in this way, the readings taken on the curved surface can be checked against those taken by the conventional method on the flat side. Too, it is often possible to obtain readings on rough stones that have shiny areas, either crystal faces or other plane surfaces that may be rested on the hemisphere; otherwise, a very tiny flat or curved surface may be polished on the rough for this purpose.
DETERMINING OPTIC CHARACTER AND SIGN
As light travels through a singly-refractive material, it is slowed down equally in all directions and remains more or less, unchanged when it leaves the stone. Thus, for a singly refractive stone, the refractometer reading on every facet will be the same even when the stone is rotated on the hemi-cylinder. On the other hand, light entering a doubly-refractive stone is split into two beams vibrating at right angles to each other. This means that the use of a monochromatic light and a polarizing filter will disclose two readings through most, or all of the rotation of a doubly-refractive stone on the hemi-cylinder.
The refractometer is selective in its analysis of a gemstone since it only gives readings for the light that is traveling in a direction that is parallel to the length and surface of the hemi-cylinder. (Figure H). If a doubly-refractive stone is placed on the hemi-cylinder so that its optic axis (direction of single refraction in a D.R. stone) is in the direction being analyzed by the hemi-cylinder, only one reading will be seen. As the stone is turned on the hemi-cylinder, changing the direction being analyzed, two readings will become visible. Figure I illustrates this. The arrow inside the stone in part A represents the optic e axis direction of the stone as it is rotated on the hemi-cylinder and part B shows the resultant readings for each position. These readings can be plotted on a graph, as shown in part C which then can be studied to determine whether the stone is uniaxial or biaxial, positive or negative in sign.
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 is called its OPTIC CHARACTER. Optic sign (positive or negative), a division of optic character, is defined briefly in the following table. 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 a the classification meaningful in identification.
This table gives a breakdown of the different divisions and subdivisions of optic character.
| Singly Refractive (Isotropic) |
Amorphous Cubic |
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| Doubly Refractive (Anisotropic) | Hexagonal Tetragonal | Uniaxial (1 optic axis) |
Positive Uniaxial, with the lower R.I. constant and the higher variable |
| Negative Uniaxial, with the higher R.I. constant and the lower variable |
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| Orthorhombic Monoclinic Triclinic |
Biaxial (2 optic axis) |
Positive Biaxial, with the intermediate R.I. closer low R.I. |
|
| Negative Biaxial, with the intermediate R.I. closer high R.I. |
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To plot a stone's readings, place the stone on the hemi-cylinder. Use monochromatic light and rotate a polarising filter over the eyepiece. Record the readings seen. Turn the stone a few degrees, no more than 45°, and record the readings seen at this position. Continue until the stone has been rotated at least 180°. At this point the reading pattern will repeat itself.
On a graph similar to the one shown in Figure IC, plot the low figure attained at each position of rotation and connect them with a line. Next, plot all the high figures in the appropriate positions and connect them also.
Uniaxial stones have only one optic axis which generally makes the plots of these stones simpler in appearance than plots of biaxial stones. There are essentially three types of graphs that can be obtained from a uniaxial stone. In one, the high and low readings are both constant resulting in two straight lines on the graph. This means that the facet being analyzed is cut perpendicular to the optic axis. The stone is definitely uniaxial and not biaxial, but the sign (positive or negative) cannot be determined from this facet. Simply plot the readings from another facet to determine the sign. The second type of, plot consists of one straight line representing a constant index upon rotation (the ordinary ray) and one line that curves away from and back to the first line, representing the variable index (the extraordinary) ray). If the lower numerical index is constant and the higher is variable the stone is positive in sign. If the high numerical index is constant and the lower is variable, the sign is negative. The third type of reading is one in which there is a constant index and a variable index but the two do not touch or meet on the graph. This stone could be either tint- axial or biaxial and must be tested on another facet.
Usually, both readings on a biaxial stone will vary. It is possible for a biaxial stone to have a graph with one index constant and one index varying but not touching the first index, but, as described above, this could also be the graph of a uniaxial stone and the stone should be checked on another facet. A biaxial stone has three vibration directions as opposed to a uniaxial stone which has only two (represented by the ordinary and extraordinary rays). These three vibration directions are represented as ALPHA, BETA AND GAMMA. Alpha is the lowest numerical index seen and gamma is the highest numerical index seen. Beta is between these two and is represented on a graph as the point where the two varying lines meet or the index that they have in common (see Figure J). If the value of beta is closer to the low numerical index, the stone is positive. It the value of beta is closer to the high numerical index, the sign is negative. This can be stated another way. In uniaxial stones, the sign is determined by noting which of the two readings varies. If the high numerical reading varies, the stone is positive. Similarly, for a biaxial stone, if the high numerical reading varies most, the sign is positive, and if the low numerical leading varies most, the stone is negative. Occasionally, it is possible to encounter a stone that has a beta value exactly halfway between alpha and gamma, in which case, both the high and low readings would vary by the same amount. This stone would be biaxial but without sign.
A problem also occurs when, in a biaxial stone, beta is only .001 or .002 from alpha or gamma. When this happens, the movement of alpha (if the sign is positive) will be so slight that its movement, as the stone is rotated, may not be detected. The stone would appear to be uniaxial when it is actually biaxial. Checking the stone on another facet or locating an interference figure, as described in the Identification by Refractometer will determine whether the stone is indeed uniaxial or biaxial.
Figures K and L show the possible readings that can be obtained from a uniaxial and biaxial stone. Figure K shows a hexagonal crystal and the possible readings from the different faces of the crystal. Figure L shows a crystal and the different variations of biaxial readings that can be seen.
SUMMARY
Once you have established that stone is doubly refractive, the next question is whether it is uniaxial or biaxial. This is done by using a polaroid plate to separate the high and low indices and noting the variation of these as the stone is rotated on the hemi-cylinder. Plot the readings on a graph. Connect the dots by first connecting the low readings from those obtained at each position on the hemi-cylinder and then connecting the high readings. The simplest case is when both indices remain constant. Such a stone is uniaxial. The second possibility is that both indices will vary. Such a stone is biaxial. The third cost possibility is a graph that looks like one straight line and one curved line. Here there are two types. The first type is a graph that shows the variable reading merging with the constant reading at one point. This stone is uniaxial. However, if the variable reading does not touch the constant reading, the stone could L., either uniaxial or biaxial, and another facet should be used to determine the optic character and sign. This is the second type. Just as it is a good idea to double chuck the polariscope determination of SR and DR with a dichroscope, microscope, or refractometer, it is always, a good idea to double check whether a stone is uniaxial or biaxial. The technique of obtaining a uniaxial or biaxial interference figure with the polariscope is described in later.
The determination of optic sign is quite easy for uniaxial stones. If the low numerical index is constant, the stone is positive. If the high index is constant, the stone is negative.
For biaxial stones, the determination of sign is more difficult. There are two approaches. First of all, for biaxial stones, beta is the common point of the two variable RI readings. On a plot of the variation of the high and low indices, the value at which they meet or that they have in common is the beta index. If beta is closer to the low index, the stone is positive. If beta is closer to the high index, the stone is negative. The second approach is to determine which index varies beyond the value halfway between the highest and lowest readings, or in other words, which index varies the most. If the low index varies the most, the stone is negative. If the high value varies the most, the stone is positive. Using one of these techniques, it is usually possible to determine the optic sign of a biaxial stone.
