Alexandrite is a rare variety of the mineral chrysoberyl, chemically known as beryllium aluminum oxide (\(BeAl_2O_4\)). This gemstone is prized for an extraordinary optical property that causes it to appear differently depending on the light source. The phenomenon Alexandrite is most famous for is a dramatic color change, formally known as the Alexandrite Effect. This transformation is why the stone is sometimes described as “emerald by day, ruby by night”.
The Defining Color Change Effect
The Alexandrite Effect is a pronounced shift in hue observed when the gemstone is moved between two different types of illumination. Under natural daylight, or a light source with a broad spectrum, the stone typically exhibits a green or bluish-green color. This appearance results from the stone reflecting the abundant green wavelengths present in daylight.
The transformation occurs dramatically under incandescent light, such as a candle or traditional household bulb. Under this warm, yellow-red rich illumination, the gemstone shifts its appearance to a purplish-red or raspberry hue. This dual personality demonstrates metamerism, where two colors appear to match under one light source but not another. The clarity and intensity of this green-to-red change are the primary factors determining the stone’s value.
The Scientific Basis of Selective Light Absorption
The cause of the Alexandrite Effect is rooted in the stone’s chemical structure and its interaction with light energy. Alexandrite’s color comes from trace amounts of chromium ions (\(Cr^{3+}\)) that substitute for aluminum within the chrysoberyl crystal lattice. These chromium ions dictate the stone’s color response.
The crystal structure causes the chromium ions to absorb light strongly in the yellow region of the visible spectrum, specifically around 590 nanometers. This strong absorption creates two “transmission windows” where light passes through: one in the blue-green wavelengths and another in the red wavelengths. The color the human eye perceives results from the balance between the light transmitted through these two windows.
Daylight has a high proportion of blue and green wavelengths, which passes through the blue-green transmission window. Since the human eye is more sensitive to green light, the stone appears green, overpowering the less-abundant red light. Conversely, incandescent light is rich in red wavelengths and contains less blue and green light. Under this light, the red transmission window dominates, allowing the eye to register the purplish-red color. The phenomenon is a balance between the spectral output of the light source, the stone’s selective absorption, and the sensitivity curve of human vision.
Angle-Dependent Color Variation
A separate optical property present in Alexandrite is pleochroism, which describes a color variation based on the viewing angle. Unlike the Alexandrite Effect, pleochroism is dependent only on the stone’s crystallographic orientation, not the light source. Alexandrite is a trichroic mineral, meaning it can exhibit three different colors when viewed along its three crystallographic axes.
These three distinct colors are typically green, orange or yellow, and purplish-red. The stone’s internal structure causes it to absorb light differently along these three axes, polarizing the light that passes through. This means that even under a single light source, turning the gemstone can reveal subtle shifts in hue.
Natural Occurrence and Synthetic Creation
Alexandrite was first discovered in the Ural Mountains of Russia in the 1830s, which provided the most famous examples of the gem. Today, significant natural deposits are also found in countries like Brazil, Sri Lanka, and Tanzania. The geological conditions required for Alexandrite formation, where beryllium and chromium exist together, are exceedingly rare, contributing to its scarcity.
Because of this rarity, synthetic Alexandrite is widely produced in laboratories for commercial use. These lab-grown stones are chemically identical to the natural material, sharing the same crystal structure and exhibiting the Alexandrite Effect. The most common methods for creating synthetic Alexandrite include the Czochralski process and the flux-growth method. These manufactured gems provide a more accessible way to observe the color-change phenomenon.