Pleochroism in gemstones

by Yuman Hussain

Scientific explanation of pleochronism which is an ability of the stone to absorb different wavelengths of light displaying brilliant colors when viewed from different directions.

What is Pleochroism

Have you ever wondered about the color of a gemstone? How and from where different colors originate in a stone? Well, it is through a phenomenon known as Pleochroism, a term with many might not be familiar with.

Pleochroism is derived from a combination of Greek words-'pleion' meaning more and 'chros' means color. However it does not mean multi-colored but a change of color in a stone due to movement of light through it. Pleochroism is a general term referred to the processes of dichroism and trichorism. Dichroism is found in uniaxial crystals with single optic axis and trichorism is found in biaxial crystals with two optic axis. When ordinary light falls on uniaxial crystal any given wavelength of light is absorbed except along the optic axis displaying one color in its direction and different color at other angles. These dichronic stones show two colors or shades and trichronic stones with two optic axis shows sometimes three or more shades or colors. The main cause of Pleochroism is due to absorption of particular wavelengths of light. This selective absorption leads to transmitted light, which appears as colored. Two conditions are must for this phenomenon including thickness and crystallographic structure. Secondly the chemical properties of the stone should be strong enough to see the effect.

Highly visible in doubly refracting colored stones Pleochroism is defined as an optical phenomenon in which light is focused on stone splitting in two different components - ordinary ray and extraordinary ray polarized at perpendicular angle to each other. When different wavelength of light is traveling at different speed following different directions within a stone it gets absorbed producing different colors when it leaves the stone. It makes the stone as if made up of different colors. It is basically an ability of the stone to absorb different wavelengths of light displaying colors when viewed from different directions. These colors depend upon the optical path viewed .This phenomenon can be understood through following example. If all the light except green and blue is absorbed in a crystal then it reflects green color. If a stone absorbs all light except yellow than it will display yellow color. If the stone is observed when turned from first direction to the other then color will change from green to yellow.

All stones do not show Pleochroism. For example isometric stones such as diamond, garnet, spinel and halite do not show change in color as they have same structure having same light absorbing capacities preventing light waves vibrating in different directions. Almost all transparent minerals show Pleochroism. Most of the dichronic stones are hexagonal, Trigonal or tetragonal with one structural direction along the major symmetry axis whereas trichronic stones are orthorhombic, monoclinic or triclinic with three axis of symmetry or direction that can absorb light.

Sometimes in order to display color or to make it more attractive stones are cut in such a way to show Pleochroism or to hide it. An important tool for mineral identification it is required to examine the stones displaying different pleochronic color schemes. When we use plane-polarized light under microscope a pleochroic stone changes its color. Many stones are pleochronic but sometimes color change is so negligent that it require optical instrument for detection. Some gemstones show stunning color displays. Often color change is restricted to shade changes for example from light purple to dark purple or violet. Pleochroism is strong when color change is distinct and it is said to be weak when there is only minor color change in a stone. An individual transparent crystal can show the display of Pleochroism.

Pleochroism is observed:
- If there is change in actual color that is, if a stone is seen blue or red from different positions.
- If there is change in the intensity of color that is if a stone becomes light to dark in shade.

Tourmaline is a dichronic substance reflecting two colors in which ordinary ray is absorbed and extraordinary ray is transmitted. Cordierite crystal show trichorism when light passes through parallel to three crystal axis violet, yellow or blue light is absorbed. When crystal is cut the axis edges show mixed colors of blue, yellow, violet and yellow, violet and blue. Tanzanite is also another popular diachronic stone in which there is selective absorption of different lights displaying brilliant colors ranging from blue to purple when seen from different light sources. Blue Aquamarine sometimes might show a slight color change from pink to green if viewed from different directions. Fluorpatite is a diachronic yellow mineral which turns blue when it is viewed along its axis. There are some other stones which show best examples of Pleochroism. These are:

A spherical shell of color, pleochroic halo is present around radioactive impurity in a stone. This ring or halo said to represent a site where crystal structure has got changed by the absorption of energy of radioactive alpha particles .As energy has got absorbed it produces an intense color centre on the inclusion with in a stone. These pleochroic halos are common in zircon, monazite, xenotime and apatite. The rings are distanced and identified according to the range of alpha particles with in a stone.

The beauty of a stone is enhanced when it display different colors from different angles. So next time while buying a gemstone do not forget to look at the kind of light displayed at the stone.

 
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