Alexandrite is a gemstone known for its ability to change color under different lighting conditions. It appears green or bluish-green in daylight or fluorescent light, transforming to a red or purplish-red hue under incandescent light. This striking transformation has led to its poetic description as “emerald by day, ruby by night”.
The Geological Genesis
Alexandrite, a variety of the mineral chrysoberyl, forms under specific geological conditions involving high temperatures and pressures. Its chemical composition is primarily beryllium aluminum oxide (BeAl2O4). The gemstone typically forms in metamorphic rocks, such as mica schists and gneisses, or in coarse-grained igneous rocks called pegmatites.
A minute amount of chromium within the crystal structure is responsible for alexandrite’s characteristic color change. Chromium ions replace some aluminum ions, creating color centers that absorb certain light wavelengths. This selective absorption, combined with varying light sources, causes the gem to display distinct colors. For alexandrite to form, the rare element beryllium must coincide with chromium, which typically occur in contrasting rock types. The absence of silica during formation is also important, as its presence would lead to the development of emerald.
Global Mining Locales
Alexandrite was first discovered in the Ural Mountains of Russia in the 1830s. These Russian deposits, particularly near the Tokovaya River, were historically renowned for producing alexandrite with vivid green-to-red color changes. Russia remained the primary source for many years, yielding some of the most highly valued specimens. However, significant finds have become rare, with current production often containing inclusions.
Other sources emerged globally. Sri Lanka has produced alexandrite, often found in alluvial deposits. While Sri Lankan alexandrite can be large, its color change may be less pronounced, sometimes exhibiting brownish or yellowish overtones compared to Russian stones. Brazil became a significant source after discoveries in the late 1980s, particularly in Minas Gerais. Brazilian alexandrite is known for its bluish-green color in daylight and a purplish-red shift under incandescent light, often with good clarity.
East Africa has notable deposits in Tanzania and Madagascar. Tanzanian alexandrite can display a lighter bluish-green in daylight and a color change. Madagascar’s alexandrite is found in pegmatites and metamorphic rocks, with some large gem-quality crystals occasionally recovered. Other minor sources include India, Zimbabwe, and Myanmar.
Factors Contributing to Rarity
Alexandrite is one of the rarest gemstones due to the specific geological requirements for its formation. The elements beryllium and chromium, both necessary for alexandrite to form, typically do not occur together in nature. Beryllium itself is a relatively rare element, and its co-occurrence with chromium in the correct geological environment is an infrequent event.
The formation process also demands specific physical conditions, including high temperatures and pressures, over thousands of years. Even when these conditions align, the presence of silica, a common element, can prevent alexandrite formation, leading instead to emerald. The limited number of locations worldwide where these precise conditions have occurred restricts the supply of natural alexandrite. Production from known mines has also diminished, making large, high-quality stones uncommon. This combination of specific geological demands and limited mining sources contributes to alexandrite’s scarcity and high market value.