Alexandrite is a rare variety of the mineral chrysoberyl, a beryllium aluminum oxide. Its defining characteristic is the “Alexandrite effect,” an extraordinary optical phenomenon causing a distinct color shift depending on the light source. The stone typically appears vibrant green or bluish-green in natural daylight, but transforms dramatically into a purplish-red or ruby-red hue under incandescent light. This ability to embody two different colors makes it one of the most highly prized and scarce gemstones.
The Unique Geological Requirements for Formation
The formation of Alexandrite requires a unique geological convergence of chemical elements. Chrysoberyl is composed of Beryllium and Aluminum, elements generally found in silica-rich pegmatite fluids. However, the signature color change requires Chromium, an element confined to silica-poor ultramafic rocks.
These two distinct geological environments must collide during a metamorphic event for Alexandrite to crystalize. Beryllium-rich fluids must react with Chromium-bearing host rocks, such as mica schists, under specific conditions of high heat and pressure. This contact-metamorphism scenario is rare because Beryllium and Chromium are chemically incompatible and rarely occur together in nature.
Crystallization occurs under medium- to high-grade metamorphic conditions, with estimated temperatures ranging between 500 and 650 degrees Celsius. The intense pressure, often between 4 and 7 kilobars, helps force the trace Chromium ions into the crystal lattice structure. This demanding recipe of unusual chemical ingredients and extreme geological forces explains why Alexandrite is rarer than most other gemstones.
Primary Global Sources and Historical Origin
The history of Alexandrite begins in the Ural Mountains of Russia. It was first discovered in emerald mines near the Tokovaya River in the 1830s and was named in honor of the future Tsar Alexander II. The stone’s red and green colors mirrored the military colors of Imperial Russia, establishing it as the national gemstone of the Tsardom.
The Russian deposits yielded stones with vivid green-to-red color change, setting the benchmark for quality, but these mines are now largely depleted. Following the decline of Russian production, new commercial sources emerged globally. Sri Lanka, historically known as Ceylon, became an important source, though its stones often display a less intense shift, moving from an olive-green to a brownish-red.
A major modern discovery occurred in 1987 at the Lavra de Hematita deposit in Minas Gerais, Brazil, which provided a significant new supply to the market. Brazilian stones often exhibit a bluish-green in daylight that shifts to a purplish-red under incandescent light, offering excellent clarity. Alexandrite is also mined in East Africa, particularly in Tanzania’s Tunduru region and Lake Manyara, where it occurs in phlogopite-bearing schists and alluvial deposits.
Understanding the Cause of the Color Change
The color shift, known as the Alexandrite effect, results from the stone’s chemical structure interacting with light. Alexandrite is chrysoberyl (BeAl₂O₄), and the color change is caused by trace amounts of Chromium ions (Cr³⁺) substituting for Aluminum ions within the crystal lattice.
This substitution creates an absorption spectrum where the mineral strongly absorbs light in the yellow-to-orange region, around 580 to 600 nanometers. Natural daylight is rich in blue and green wavelengths; thus, the stone transmits and reflects green light, causing it to appear green.
Incandescent light, such as from a candle or tungsten bulb, has a much higher concentration of red wavelengths and fewer blue and green ones. Because the gem absorbs the green/yellow light, the only remaining light transmitted to the eye is the red portion of the spectrum, causing the stone to appear red or purplish-red. The perceived color is determined by which wavelengths of light the stone allows to pass through under different illuminating conditions.