Alexandrite is one of the world’s most sought-after gemstones, distinguished by its striking color-change phenomenon. This optical property, often called the “Alexandrite effect,” causes the gem to appear green in daylight and shift to a red or purplish-red hue under incandescent light. Alexandrite is a variety of the mineral chrysoberyl. Its extreme rarity results from the highly improbable geological conditions necessary for its formation, limiting the locations where it is found.
The Conditions Required for Formation
The mineral chrysoberyl is chemically composed of beryllium aluminum oxide. For it to become color-changing alexandrite, a small amount of the trace element chromium must replace some aluminum atoms in the crystal structure. The extreme rarity of alexandrite stems from the fact that beryllium and chromium rarely occur in the same rock formation. Beryllium is typically concentrated in light-colored, silica-rich rocks called felsic pegmatites, which form during the final stages of magma crystallization.
In contrast, chromium is primarily found in dark, silica-poor rocks known as mafic or ultramafic rocks, such as peridotites. Alexandrite only forms in the narrow, geochemically improbable boundary where beryllium-rich fluids from a pegmatite intrusion react with chromium-rich country rock under intense heat and pressure. This interaction occurs within metamorphic rocks, like mica schists, creating the unique environment where the two elements can combine. The original Russian deposits are a geological model for this type of metamorphic contact zone formation.
Historic and Primary Source Locations
The original and most celebrated source of alexandrite is the Ural Mountains in Russia, where the gemstone was first discovered in 1834 near the Tokovaya River. Russian alexandrite from this region is considered the benchmark for quality, displaying a sharp, intense bluish-green in daylight and a vivid purplish-red under incandescent light. The Russian mines were largely depleted by the turn of the 20th century, cementing their historical importance rather than their current commercial output.
Brazil emerged as a major modern source of alexandrite following the discovery of deposits in Minas Gerais state, particularly the Hematita mine, starting in the late 1980s. Brazilian stones typically exhibit a slightly different color profile, often appearing blue-green in daylight and changing to a deep raspberry or purplish-red under artificial light. This region has been the most significant supplier of fine-quality alexandrite in recent decades, although production quality has declined.
Sri Lanka, historically known as Ceylon, has also been a long-time producer of chrysoberyl, with alexandrite found in alluvial deposits near areas like Ratnapura. The Sri Lankan material often yields larger crystals than the Russian or Brazilian sources. The color change in Ceylonese alexandrite is frequently less pronounced, typically shifting from a yellowish-green to a brownish-red.
Secondary and Emerging Global Deposits
Beyond the primary sources, several other regions contribute to the global alexandrite supply, often with smaller yields or less dramatic color shifts. In East Africa, Tanzania has produced fine material, particularly from the Tunduru and Lake Manyara areas. Tanzanian alexandrite can exhibit a strong green-to-red transition, sometimes comparable to the Brazilian stones, though the gems are often smaller.
India is another notable source, with deposits found in the state of Andhra Pradesh, including the Araku Valley. Indian alexandrite typically shows a greenish-gray color in daylight, shifting to a brownish-red or purplish hue. While many stones have a weaker color change, certain rare specimens can rival the quality of the Russian and Brazilian material.
Other African nations also contribute, including Madagascar, particularly the Mananjary and Ilakaka regions, and Zimbabwe. Madagascar’s deposits have yielded good quality material, sometimes of significant size. Alexandrite from Zimbabwe’s Novello deposit is generally darker, making it more suitable for smaller stones that show an intense color change. These secondary locations help meet the persistent demand for the rare color-changing gem.