Alexandrite is a rare gemstone known for its shift in color depending on the light source. This optical property, sometimes described as “emerald by day, ruby by night,” has captivated observers since its discovery. The gem’s dual nature made its initial identification a challenge, contributing to the mystique surrounding its origins. The scientific explanation involves the stone’s chemical structure and the spectrum of light.
Unveiling the Discovery: Date, Location, and Naming
The discovery of Alexandrite is closely tied to Imperial Russia, specifically in the emerald mines of the Ural Mountains. The initial find occurred around 1830 in the Tokovaya River region, followed shortly by scientific identification. Finnish mineralogist Nils Gustaf Nordenskiöld examined the first samples, initially mistaking them for emeralds due to their green color in natural light.
Nordenskiöld was perplexed by the stone’s high hardness, which was greater than typical emeralds. He later confirmed the stone was a new variety of chrysoberyl after observing its color change to red under candlelight. The gemstone was officially named Alexandrite in honor of the then-heir apparent to the Russian throne, Tsarevich Alexander Nikolaevich, who would later become Tsar Alexander II. The naming is often linked to his sixteenth birthday on April 17, 1834.
The Unique Science Behind the Color Change
The color-shifting property is known as the Alexandrite effect, an optical phenomenon resulting from the gem’s selective light absorption. This effect is a specialized form of metamerism, where the color observed depends on the spectral composition of the light source. The shift is caused by trace amounts of the element chromium (Cr3+) within the crystal structure.
Chromium acts as a chromophore, strongly absorbing light in two narrow regions: the dark blue/violet and the yellow parts of the visible spectrum. Under natural daylight, which is rich in blue and green wavelengths, the gem reflects the unabsorbed light, causing it to appear green or bluish-green. Incandescent light, conversely, contains a greater proportion of red wavelengths.
Since the chromium chromophore does not absorb much red light, the stone reflects the abundant red wavelengths, causing it to display a purplish-red or raspberry color. The balance between the transmitted red and green light is delicate, meaning a small change in the light source can trigger a change in the stone’s perceived color. The intensity of this shift is a primary factor in determining the value of a natural Alexandrite.
Defining Alexandrite: Chemical Composition and Classification
Alexandrite is a variety of the mineral chrysoberyl. The base chemical composition of chrysoberyl is beryllium aluminum oxide, represented by the formula BeAl2O4. Alexandrite is the chromium-rich variant of this mineral, where a small percentage of chromium ions substitute for aluminum ions in the crystal lattice.
The presence of chromium is what distinguishes Alexandrite from ordinary, colorless chrysoberyl. This mineral is considered quite durable, ranking at 8.5 on the Mohs scale of mineral hardness. Alexandrite forms crystals within the orthorhombic crystal system, which contributes to its characteristic structure.