pleokroism

Pleokroism is a physical phenomenon observed in certain minerals and gemstones where the color of the stone appears to change depending on the angle from which it is viewed. This optical effect is caused by the differential absorption of light by the mineral's crystal structure. Different crystallographic axes within the mineral can absorb light of different wavelengths, leading to a variation in perceived color when light passes through the stone at various orientations.

The intensity and nature of the pleokroism vary greatly among different minerals. Some minerals exhibit strong pleokroism, displaying distinct and dramatic color changes, while others show only subtle variations. This property is particularly important in gemology for the identification and grading of gemstones. For instance, a sapphire might appear blue from one angle and a lighter blue or even greenish-blue from another. Similarly, a tourmaline can exhibit a range of colors like red, pink, and brown depending on its orientation.

Gemologists use a dichroscope, an optical instrument, to observe and measure pleokroism. By looking through the dichroscope, one can often see two distinct colors simultaneously, representing the two most prominent pleochroic colors of the gemstone. This characteristic is a valuable tool in distinguishing between similarly colored gemstones and in identifying treatments that may have altered the stone's natural pleochroic properties. The phenomenon is directly related to the mineral's crystal system and the arrangement of its atoms.