In isotropic environments, the absorption of light is the same in all directions. In contrast, in colored anisotropic crystals the absorption depends on the direction, in which the vibrations of polarized rays take place. The change in the color of the crystals depending on the direction of vibrations of the light rays is called multicolor, that is, pleochroism.
The cut is adapted to the change of pleochroic colors: a — rubinu, b - tourmaline.
The ruby shows a pronounced pleochroism. If you are looking at this stone in the direction of the principal axis, its color is darker than that perpendicular to the axis. Tourmaline is also pleochroic, but the color absorption ratios are different from that of ruby. Tourmaline is opaque to the ordinary ray. The dark color is perpendicular to the main axis, and it is bright in the direction of this axis. This is the reason for using a different cut for these stones. Cubic cube cut from cordierite has different colors in three perpendicular directions: blue-gray, yellow, indigo blue. Similar color differences, depending on the direction, occur in Kunzite and Tanzanite.
Optically uniaxial bodies, e.g.. ruby or tourmaline, they have - when we look at them in transmitted light - 2 main indexes of refraction and their corresponding 2 main colors. Such bodies are called dichroic, and the phenomenon itself - a dichroism. Optically biaxial bodies, e.g.. cordierite or kuncite, with three refractive indices, they show 3 main colors. Such bodies are called trichroic bodies, and the phenomenon - trichroism.
Pleochroism is a general term, embracing both dychroism, as well as trichroism.
Colored optically isotropic minerals do not show pleochroism. Uniaxial minerals show no color change in sections perpendicular to the optical axis, because in this direction they behave as optically isotropic bodies. In uniaxial crystals, two essentially different colors appear in cross sections that are parallel and oblique to the optical axis. Similarly, in optically biaxial crystals, no color change is found in sections perpendicular to one of the two optical axes.. These crystals have three different colors or shades of color in three perpendicular directions.
However, the color differences that occur frequently are not clear enough to’ to find them with the naked eye. The instrument shown in the picture, called a dichroscope, is then helpful.
Dychroscope: a - structure,, b - general view: 1 — romboedr kalcytowy, 2 - lens, 3 - cork; And - the eye of the observer, II - dychroskop, III - crystal (test stone), IV - two images of the dychroscope window as seen through the eyepiece.
Dychroscope, also called from the inventor, Viennese mineralogist and geologist W.. Haidingera (1795—1871), Haidinger's magnifying glass, it is a small instrument, which denotes pleochroism of minerals. With this device, the individual colors are observed instead of the mixed color of the pleochroic mineral. The dychroscope consists of an elongated rhombohedron of clear calcite, placed in a metal tube with a circular cross-section 1. There is a square hole at one end, on the other, facing the eye, weak lens and round hole. The sizes of the calcite rhombus and the square hole are so selected, that the small square hole at the end of the metal frame gives two pictures side by side due to the double refraction of light. If we place a colored and transparent crystal with a clear pleochroism in front of this square hole, this is when looking through the dychroscope from the lens side 2 we will see two differently colored square fields. The test stone is placed in front of the dychroscope in this way, that it can be rotated around the vertical axis by means of a metal rod. The color differences are the greatest, when the directions of light vibrations in the calcite coincide with the vibrations in the examined crystal.
This phenomenon occurs in all directions, in which the light is refracted twice. Towards a single refraction of light, i.e.. in the direction of the optical axis, both fields have the same color.
In some cases, e.g.. in sapphires or rubies, especially the darker ones, both fields have the same color - blue or red, but of a different shade. In other cases, e.g.. in Alexandrite, when turning the stone, completely different colors are visible - red, green and orange (two at the same time).
The observation of stones with a dychroscope allows for a quick differentiation of an optically anisotropic body, double breaking the light, from isotropic, i.e.. a crystal belonging to a regular or amorphous body, e.g.. glass used to mimic gemstones. This is how a ruby can be distinguished from a similar red spinel or garnet, which crystallize in a regular pattern, similarly, sapphire from a regular blue synthetic spinel, etc..
The most appropriate light, which should be used to observe gemstones with a dichroacope, there is daylight.