The subject of pleochroism (dichroism and trichroicsm) was disccused briefly in the Colored Stone Course. Additional references to the colors displayed by the various pleochroic stones were contained in the colored-stone lessons; more complete tables of dichroism accompany this assignment.
The Cause of Pleochroism Colors.
We have learned that when light passes through a colored stone along a direction other than one of single refraction (i.e., an optic axis its vibration directions are confined to planes at right angles to one another. Light vibrating in these two directions is transmitted at different speeds, and it also may be subject to differences in selective absorption within these two planes. For example, light transmitted through a ruby in a direction of double refraction is not only polarized in two sets of planes at right angles, but the two are subject to a different absorption. The result is two differently colored beams of light emerging from the stone: orangey red and purplish red (Figure A. Of course, to the eye, the two colored beams are not usually visible separately, since the eye is receptive to light vibrating in all planes.
How Pleochroism is Revealed
The strength of pleochroism in a stone that displays the phenomenon is related to the depth of color of the stone; i.e., a colorless stone displays none, whereas a very dark-colored stone displays it to a maximum degree.
To reveal the presence of pleochroism to best advantage, the light vibrating in each plane must be viewed separately. Although it can be observed in the polariscope, the ideal instrument for this the dichroscope
Although dichroscopes may be made in a range of sizes and different materials may be used in their construction, the usual instrument is a small tube employing a special polaroid film as the key element. The polaroid film, about .011 inch thick, was carefully researched and created for use in the GIA dichroscope and consists of an optically clear base onto which a polaroid material has been placed such that one half of the film transmits light vibrating in one direction while the other half transmits light vibrating at a 90° angle to the first.
The film is placed at one end of the tube against the window; at the other, or eyepiece, end is located a small lens with a focal length such that it focuses about one-quarter of an inch beyond the polaroid film (Figure B). When using the dichroscope, the window will appear with a line clearly visible through its center. The area on each side of the line represents light vibrating in a simple direction and the two sides have vibration directions at right angles to one another. Thus, a dichroic gem viewed through the dichroscope in such a position that each side of the window coincides with, one of the gem's vibration directions will show different colors in the two halves. The great advantage of the dichroscope over the polariscope as a detector of pleochroism lies in the fact that the two colors are seen side by side.
Another form of dichroscope construction -- which has been used in the past — employed transparent calcite of the in the past -- employed transparent calcite of the Iceland spar variety as the key element. Due to the difficulty in obtaining fine quality optically clear material, as well as the greatly increased cost of same, manufacture has become prohibitive, and this form of the instrument has been discontinued.
The dichroscope is not difficult to use, but some precautions are necessary for best results. It is best to use a light source as devoid of color as possible. The stone should be held in tweezers or between thumb and forefinger, so that it may be turned readily to permit examination in a number of directions. It should be held before a strong but diffused light source, such as a substage lamp with a frosted-glass diffuser; the light aperture at the FRONT of the Illuminator Polariscope (Figure C); or an overcast sky. If the polariscope light is used, be sure to avoid holding the stone over the polarizer, for the very strong dichroism of the Polaroid will be all that you see. Another precaution: Make sure that the only light reaching the window is the light that has been transmitted through the stone. Be sure the light has not been reflected from any facet, thus being partly polarized.
Suppose you are examining a gem through the pavilion direction and in doing so it is turned so that an internal reflection from the table is the light being examined. Reflection from a plane surface causes partial polarization, so one window could, be brighter than the other and the result could be mistaken for weak dichroism. Partial polarization by reflection is possible in a singly-refractive stone, so that the danger is obvious. To avoid such an error, be sure that the light is passing directly through the stone without internal reflection from a facet. For this reason, the dichroscope should not be used on a stone lying on a flat surface with light directed down on it from the opposite side.
Fluorescent tubes emit unpolarized light when the center of the tube is viewed, but light coming from the edges is partially polarized. As a result, a dichroscope user may see very weak dichroism in a stone examined under such a lamp when actually none exists. These problem light sources would not cause an error involving a finding of distinct or strong dichroism with two different hues seen, but slight differences of tone or intensity, as expected in very weak to weak dichroism, could be assumed mistakenly to be present. On the other hand, weak or very weak dichroism is often overlooked by an inexperienced tester, because he examines the stone in one direction only.
Another common error is to examine a stone in a direction in which total reflection has prevented light from passing through the stone. Obviously, light must enter the stone and be polarized for dichroism to occur.
Even if transmitted light is studied with the dichroscope, it is necessary for the instrument to be rotated so that its planes of polarization coincide with those of the stone for the full dichroic difference to be apparent. This usually requires both rotation of the instrument and turning of the stone, to bring about the conditions for optimum color-difference display.
To detect pleochroism, the polariscope is used differently from its use in distinguishing single from double refraction. In this situation, only one direction of transmission is cut out. This is accomplished by turning the analyzer so that its transmission direction is parallel to that of the polarizer (the direction of greatest light transmission).
When a doubly-refractive stone is examined in the polariscope's light position, the light passing into it is polarized by the first Polaroid plate. During rotation of the stone in a position other than one in which an optic axis is perpendicular to the polarizer (parallel to the line of sight), the polarized light will vibrate in a direction corresponding to one of the gem's vibration directions every 90°. The characteristic absorption for that direction will be visible when the polarized beam coincides with it; the other will be visible after a 90° rotation of the stone. As the pleochroism stone is rotated, it shows first one color and then a second.
The fact that colors cannot be observed side by side, as in the dichroscope, makes it difficult to detect very weak pleochroism. On the other hand, only a small number of gem varieties display strong pleochroism; thus, its very existence is a strong indication of the species and variety.
Interpretation of Results.
Figure D illustrate why some doubly-refractive stones (for example, those that are uniaxial)display a maximum of two colors. In a uniaxial stone there are only two sets of vibration planes: those parallel to the optic axis and those at right angles to it. In the illustration, the colors associated with the two planes of vibration are designated as A and B. Regardless of the direction of observation through the specimen, as long as the optic axis is perpendicular to the line of sight, the two planes of vibration will always be present.
The biaxial stone illustrated basically in Figure E is not symmetrical about any single crystal axis with respect to the remaining two axes; instead, if the principal directions are examined, three unequal dimensions containing a total of three distinct planes of vibration will be revealed. Two of the three are present in each direction. Each of these is capable of imposing its own characteristic selective absorption of light vibrating in it. However, it is obvious that only two planes can be observed at any one time, and a maximum of three colors could be obtained from a complete examination. Biaxial stones may show no perceptible difference in absorption in two of the three vibration directions, or none in all three. Thus, biaxial stones may show one, two or three colors in the dichroscope.
The fact that a stone is dichroic does not necessarily mean that dichroism will be evident when the gem is first viewed. The vibration directions of the instrument may be close to 45° to those of the gemstone. In such a situation, some light from both planes of vibration being observed will enter each of the vibration planes of the calcite in the dichroscope and the two windows will show the same color. Only after rotating the instrument so that its vibration directions correspond to those of the stone does the full dichroic difference in color become evident. When using the polariscope, the stone has to be rotated at least 90°, and usually more, before each of the dichroic colors can be seen. In addition, the rotation cannot be in the plane at right angles to the optic axis, for that is the direction of single refraction. If the stone is examined parallel to the optic axis, only one color will be visible.
Detecting Pleochroism with the Unaided Eye
Very weak to weak dichroism usually is detectable only with the dichroscope, the polariscope being suitable only for analyzing distinct to strong dichroism. However, in larger stones strong, and sometimes distinct, pleochroism often can be detected by the unaided eye. It is particularly noticeable when the selective absorption in one vibration direction imparts a strong color and the other permits white light to pass almost without effect; i.e., colorless, a very light hue, or in cases where contrasting colors are observed. Dichroism is visible to the unaided eye usually as a noticeable directional difference in color. For example, a stone may appear to be deep in color in one direction and very pale in another direction. Tourmaline often shows this effect. An appearance resembling uneven coloration may also result in very strongly pleochroism stones such as andalusite.
Such strong pleochroism often is masked by cutting, but usually it is readily apparent in the crystal. Cleavage pieces of kunzite are deeply colored parallel to the cleavage direction and very pale across it. This makes it essential that the stone be cut so that the direction of strong color is perpendicular to the table. Other stones that show one nearly colorless direction are blue zircon and benitoits. Zircon changes from strong blue to colorless and benitoite from strong blue to pale gray. Brown and green tourmaline and brown korncrupine have one direction that is almost black; in any other direction, they may be quite transparent and attractive in color. In any event, the strong difference in color is apparent if the stone is turned.
Although the twin colors do not change from nearly colorless to a deep color or from a color to black in ruby, sapphire and emerald, these stones do show a distinct color change, depending on the direction of view. Natural ruby almost always is cut so that the deep violetish red color is visible through the table. To most persons, this makes the natural much more attractive than when it is cut in the usual manner of synthetic ruby: with the orangey-red color visible through the table. A fine blue sapphire has a direction in which the color transmitted is greenish blue, a color that most persons find distinctly less appealing than the violetish blue usually seen through the table in a properly oriented stone.
With polariscope and dichroscope available, why be concerned with pleochroism to the unaided eye? Two factor make it important: its bearing on the cutting of stones to bring out their most attractive characteristics, and its value in identification without instruments. Recognition of such things at auctions, or in other situations where testing is not feasible, enables the trained gem man to buy advantageously those worthwhile stones that go unnoticed by others.
The use of Pleochroic findings in Identification.
How valuable are the findings made with the dichroscope? Dichroism is seldom used as a single test by which a stone is identified conclusively, yet it has great value. A number of stones have pleochroic colors that are distinctive enough to be almost a safe criterion of identity. The dark-violetish-blue and light-greenish-blue colors of sapphire and synthetic sapphire, the dark-violatish-red and light-orangey-red colors of ruby and synthetic ruby, and many other pairs of colors listed in the table that follows are sufficiently distinctive so that only one confirmatory test is likely to be needed to establish identity beyond question. It is a very valuable check test to confirm double refraction. Moreover, it is a very intriguing property to demonstrate to customers or friends. Dichroism is not used to full advantage in testing by many people. It is well worth the effect to become adept in the use of the dichroscope or Polariscope for this purpose.
The symbols S, M, W and VW signify strong, moderate, weak and very
weak pleochroism. When little or no color difference is detectable
between two of the three directions in a biaxial stone, only two colors
are given. Colors may vary in tone from those described, depending on
due and depth of body color. Very weak pleochroic colors have little, if
any, diagnostic value; therefore , they are not listed. Colorless
stones, opaque stones, and those belonging to the cubic system, of
course, are not included.
|Pink & Red Gems|
|Morganite (M)||Light red and light voilet.|
|Corundum (nat. & syn.)|
|Ruby (S)||Purplish red and orangey red.|
|Zircone (M)||Reddish purple and purplish brown|
|Orange & Brown Gems|
|Golden (W)||Greenish yellow and yellow|
|Brown (W)||Brownish yellow and greenish yellow|
|Corundum (nat. & syn)|
|Sapphire (S)||Yellow-brown or orange and colorless|
|Idocrase (S)||Light brownish green and brownish orange|
|Phenakite (M)||Colorless and brown|
|Citrine (W-M)||Yellow-brown and yellow|
|Smoky (W)||Brown and reddish brown in dark stones; light and darker yellow-brown in lighter stones|
|Tourmaline (S)||Brown and yellow-green.|
|Zircon (W-M)||Purplish brown and brownish yellow.|
|Beryl (W)||Yellow and light yellow.|
|Corundum (nat. & syn.)|
|Sapphire (W- M)||Yellow and light Yellow.|
|Phenakit(M)||Colorless and orange-yellow.|
|Synthetic rutile (VM)|
|Tourmaline (M)||Light yellow and dark yellow.|
|Zircon (W)||Light yellow and yellow.|
|Beryl (nat. & syn.)|
|Emerald (S)||Green and blue-green.|
|Corundum (nat. & syn.)|
|Sapphire (W- M)||Green and light yellowish green.|
|Diposite(W)||Light green and dark green.|
|Idocrase (W)||Yellow-green and yellow-brown.|
|Tourmaline (S)||Blue-green and yellow-green to brown-green.|
|Apatite(W-M)||Yellow and blue.|
|Benitotite (S)||Colorless or very pale gray-blue and very dark.|
|Aquamarine (W-M)||Blue and light blue to blue-green.|
|Corundum (nat. & syn.)|
|Sapphire||Violetish blue and greenish blue.|
|Idocrase (W-M)||Dark blue and light blue to colorless.|
|Synthetic rutile, (W)||Light blue and slightly darker blue.|
|Tourmaline (S)||Lighter and darker blue.|
|Zircon (S)||Blue and grayish yellow to colorless.|
|Violet & Purple Gems|
|Benitoite (S)||Reddish gray and purple.|
|Corundum (nat. & syn. )|
|Sapphire (S)||Violet and orange.|
|Amethyst||Violet or purple and gray-violet.|
|Tourmaline (M-S)||Light purple and purple.|
|Zircon (W -M)||Blue-violet and greenish brown.|