When a beam of white light enters a colored doubly-refractive gem material, either synthetic or natural, each of the beams, in addition to traveling at different velocities, may suffer a characteristic absorption of certain of its component wavelengths and emerge as a different color; this is called PLEOCHROISM (from the Greek "pleo," meaning MORE, and "chroma" meaning COLOR). Regardless of the direction observed, with the exception of the one direction or angle refraction, stones of the tetragonal and hexagonal systems will display only two pleochroic (pronounced PLEE-oh-KRO-ik) colors and are said to exhibit dichroism(pronounced DYE-ho-izm) or to be DICHROIC (pronounced dye-KRO-ik). Stones of the orthorhombic, monoclinic and triclinic systems may show a total of three distinct pleochroic colors, but only two will be visible from ANY ONE DIRECTION; these gems are said to exhibit TRICHROISM (pronounced TRY-kro-izm) or to be TRICHROlC (pronounced try-KRO-ik). Stones of the tetragonal and hexagonal systems will never show more than two colors, but the remaining systems may show a total of two or three colors, for in some gems there is no discernable difference in color in two of the three directions.
Tests for Double Refraction
Several methods may be employed in order to determine whether a transparent gem possesses double refraction or not. One of the simplest is to look through the table of a stone with a loupe and observe whether the edges between the back facets appear as single or double images. Through practice, one may become sufficiently proficient to distinguish the difference between ruby, which has a low birefringence, and red spinel and red garnet, both of which are singly refractive. This is also an easy way to distinguish colorless zircon or synthetic rutile, which are doubly refractive, from diamond, which is singly refractive. Since glass is singly refractive, the test may also sometimes be helpful in distinguishing between genuine doubly-refractive stones and glass.
Because one might look along one of the two possible directions of single retraction in a doubly refractive stone, the stone should be examined in at least three directions. Examination in additional directions may also assist, because the doubling of the edges increases from none, parallel to the optic axis (the direction of single refraction), to a maximum at right angles to the optic axis. Also, the distance between the twin images of the facet edges increases with an increase in the depth of the stone. When first seen by a student, the doubled facet edges are often likened to the appearance of railroad tracks.
THE POLARISCOPE (pronounced po-LARE-uh-scope) is an inexpensive and very satisfactory instrument for the detection of double refraction. It employs Polaroid (trademark) plates to accomplish the necessary polarization. Polaroid is made by bonding on plastic film a very strongly dichroic material composed of tiny crystals in parallel positions, optically. Thus they behave as a single large crystal plate, the dichroism of which is so strong that light in one of the two vibration directions is almost totally absorbed, whereas the other direction pusses light readily.
The polarizer's, which are attached to the instrument, are rotated with respect to one another until little or not light is allowed to pass, The gem is then placed between the polarizer. If the gem is doubly refractive and is not being observed parallel to an optic axis as it is rotated, it will appear dark at every 90° position and light at the intermediate positions. Amorphous materials and gems of the cubic system are not doubly refractive and do not affect light in this manner; they remain dark in ALL positions when viewed between the polarizer. In order to be sure that observations are correct, it is necessary to turn the gem through several positions.
Some amorphous materials and gems of the cubic system show ANOMALOUS (pronounced ah-NAHM-ah-Iuss), or false, double refraction ( which is caused by internal strain. Practice with the use of the polariscope soon enables one to distinguish between anomalous double refraction and true double refraction.
Tests for Pleochroism
Pleochroism is most easily observed with a DICHROSCOPE (pronounced DYE-kro-scope), a small instrument that is capable of separating the two polarized beams so that they can been seen separately. Looking through a dichroscope, one sees twin images of the small opening in the opposite end of the instrument. These double images are produced by a piece of optical quality calcite when a dichroic stone is viewed, the two squares may appear different colors. Since the two squares appear side by side, it is possible to observe even weak dichroism. The colors of trichroic gems may be observed by first viewing one pair of colors and then shifting the gem with respect to the dichroscope and viewing a second pair, one of which is repeated from the first. It is necessary to observe a gem along at least three directions before deciding it is not pleochroic, in order to be sure that the first two observations were not made parallel to the optic axes of a biaxial stone. Both the strength of the pleochroism observed and the colors seen are taken into consideration in determinations. Degrees of pleochroism are expressed as very strong, strong, distinct, weak, and very weak.
Pleochroism may also be observed with the polariscope, by turning the upper Polaroid so that maximum light is transmitted and rotating the gem between the polarizer. If it is a pleochroic gem and is not being observed parallel to an optic axis, it will change color during the rotation. As a rule, weak pleochroism is more difficult to observe with the polariscope than with the dichroscope, but even fairly weak color differences are evident to an observer who has had sufficient practice in using a polariscope.
The primary value of the observation of pleochroism is that its presence is PROOF OF DOUBLE REFRACTION: The polariscope detects doubly-refractive gems even if they are colorless or very pale in color; this determination is not possible with the dichroscope. However, the dichroscope does show the exact NATURE of the pleochroic colors, an observation that is sometimes of value in identification.
Moreover, the dichroscope does not show pleochroism in those ordinarily singly-refractive colored materials that exhibit anomalous double refraction. As stated previously, experience permits the observer to distinguish between true and anomalous double refraction in the polariscope, but this distinction is sometimes difficult. Generally speaking, the polariscope is a more valuable testing instrument than the dichroscope; for best results in gem testing, however, it is advisable that the two be used in conjunction with one another.
In his early observations with the dichroscope, the student gemologist usually expects to see a great difference between the pleochroic colors. New students often decide that gems exhibiting weak or even distinct pleochroic colors have no pleochroism whatever, because the difference between the colors is not obvious. The ability to detect weak and very weak pleochroism increases with practice, as does the ability to see slight differences of color in apparently colorless diamonds. Often one color will be simply a trifle lighter or darker or of a slightly different hue.