Alexandrite is a rare variety of the mineral chrysoberyl, celebrated for its ability to change color under different types of illumination. This optical phenomenon, often called the “Alexandrite Effect,” causes the gem to appear green or bluish-green in daylight. The stone transitions to a distinct red or purplish-red hue when viewed under incandescent light. This shift is a direct consequence of the stone’s unique chemical structure and the physics of light absorption.
The Mineral Foundation
The structure of alexandrite is based on the mineral chrysoberyl, an aluminate of beryllium (BeAl2O4). This mineral forms in the orthorhombic crystal system, meaning its structure has three perpendicular axes of different lengths. The crystal lattice is composed of oxygen ions arranged in a closely packed configuration.
Within this oxygen framework, beryllium and aluminum ions occupy specific sites. The aluminum ions (Al3+) are situated in octahedral coordination sites, meaning each aluminum ion is surrounded by six oxygen ions. This rigid, ordered arrangement provides the foundational structure that enables the color-change mechanism.
The Chromophore Element
The presence of a specific impurity differentiates alexandrite from common chrysoberyl. The element responsible for the color change is chromium, specifically in its trivalent ionic state (Cr3+). This trace element substitutes for a small fraction of the aluminum ions within the crystal lattice.
Chromium is a powerful coloring agent known as a chromophore, typically comprising less than one percent of the gem’s total composition. This substitution is possible because the Cr3+ ion is chemically similar in size and charge to the Al3+ ion it replaces. The placement of chromium in the octahedral site dictates its unique interaction with light, allowing it to act as a selective light filter. This arrangement is why chromium creates the green of emeralds and the red of rubies, despite different surrounding crystal structures.
The Physics of Selective Absorption
The color change is rooted in how the Cr3+ ions absorb energy from the visible light spectrum. The electrons in the chromium ion are held in specific energy levels by the surrounding oxygen atoms, a principle described by ligand field theory. When light strikes the stone, the electrons absorb photons matching the energy difference between these levels, causing them to jump to a higher energy state.
In alexandrite, absorption is strong in two areas: the dark blue and yellow regions of the spectrum. The primary absorption band is centered around 580 nanometers, in the yellow part of the visible spectrum. By absorbing the central part, the stone transmits light at both ends of the visible range: a window in the blue-green area and another in the red area.
This selective absorption leaves the stone at a visual “tipping point” because it transmits roughly equal amounts of green and red light. The intensity of the transmitted red and green light are nearly balanced. The final color perceived is highly sensitive to minor changes in the light source composition.
Light Source Dependency
The final perceived color of alexandrite is determined by the spectral composition of the light source. Daylight, or light from a fluorescent bulb, is considered “balanced” or “cool” light because it contains a high proportion of blue and green wavelengths. When the stone is exposed to this light, the abundance of blue-green energy passing through the crystal’s blue-green transmission window is greater than the red energy.
Since the human eye is highly sensitive to light in the green region, the increased presence of these wavelengths causes the stone to appear green or bluish-green. Under artificial sources like incandescent bulbs, candlelight, or firelight, the spectral output is different, being richer in red and yellow wavelengths while lacking blue and green.
In this “warm” light, the dominant red wavelengths easily pass through the stone’s red transmission window. Although the stone still transmits green light, the light source provides far less green energy. This shift in the ratio of transmitted red to green light tips the balance, and the stone’s color perception changes to reddish-purple or ruby red.