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Alexandrite is a rare and highly valued variety of the mineral chrysoberyl, renowned for its distinct pleochroism and strong color-change effect (alexandrite effect). It appears emerald green in daylight or fluorescent light and raspberry red to purplish-red in incandescent light. This phenomenon is due to the unique absorption of light across the visible spectrum, specifically the strong absorption in the yellow region, caused by the presence of chromium (Cr3+) ions substituting for aluminum (Al3+) in the crystal lattice. The intensity of the color change is a primary factor in its value.
How to Identify
- Color
- Emerald green to bluish-green in daylight/fluorescent light; raspberry red to purplish-red in incandescent light. The strength of the color change is a key identifier.
- Luster
- Vitreous (glassy) to sub-adamantine.
- Texture
- Smooth, often with striations on crystal faces.
- Crystal Form
- Orthorhombic, typically forming tabular or prismatic crystals, often twinned, particularly as pseudohexagonal trillings (three crystals intergrown at 120 degrees).
- Cleavage
- Distinct on {110}, but rarely observed in gem material due to its toughness.
- Geological Environment
- Found in pegmatites, mica schists, and contact metamorphic deposits. Also occurs in alluvial deposits derived from these primary sources.
Key Facts
- Hardness: 8.5 (Mohs scale)
- Specific Gravity: 3.70-3.78
- Crystal System: Orthorhombic
- Color: Green to bluish-green (daylight/fluorescent); Red to purplish-red (incandescent)
- Luster: Vitreous to sub-adamantine
- Transparency: Transparent to translucent
- Fracture: Conchoidal to uneven
- Cleavage: Distinct on {110}, but rarely observed
- Composition: Beryllium aluminum oxide with chromium impurities (BeAl2O4:Cr)
Quick Check
- Color: Green in daylight, red in incandescent light
- Luster: Vitreous
- Streak: White
Physical Characteristics
- Crystal Habit: Tabular, prismatic, often twinned (pseudohexagonal trillings)
- Cleavage Type: Distinct on {110}, but typically not seen in gem material due to toughness
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to sub-adamantine
Formation
Alexandrite forms in pegmatites and mica schists, and also in contact metamorphic deposits associated with beryllium-rich igneous rocks. The presence of both chromium (Cr3+) and iron (Fe3+) in the crystal lattice, along with beryllium (Be) and aluminum (Al), is crucial for its formation. It typically crystallizes from residual magmatic fluids or during regional metamorphism where these specific elements are concentrated.
Usage
Primarily used as a highly prized gemstone due to its rare color-change phenomenon and scarcity. It is a collector's stone and is also used in high-end jewelry.
Age Distribution
Precambrian to Cenozoic, depending on the host rock formation. Most significant deposits are associated with ancient metamorphic and igneous rocks.
Where to Find
Ural Mountains, Russia
Historically the most famous and original source, known for producing alexandrites with a very strong and distinct color change. Discovered in 1830.
Minas Gerais, Brazil
Significant producer of alexandrite, often with good color change, though sometimes less intense than Russian material. Found in pegmatites and alluvial deposits.
Sri Lanka (Ceylon)
Known for producing larger alexandrite crystals, often with a less pronounced color change, appearing more brownish-green to brownish-red. Found in gem gravels.
Andrahoy, Madagascar
A more recent source, producing alexandrites with a good color change, often with a bluish-green to purplish-red shift.
Tanzania
Various localities produce alexandrite, often with a good color change, sometimes with a slightly yellowish-green hue in daylight.
Finding Tips
Light Source Observation
Always observe potential alexandrite under multiple light sources: natural daylight, fluorescent light, and incandescent light. The color change is the definitive characteristic.
Hardness Test
Alexandrite has a hardness of 8.5 on the Mohs scale, making it harder than quartz (7) and topaz (8). This can help differentiate it from softer imitations.
Specific Gravity
Its specific gravity (3.70-3.78) can be a useful diagnostic tool, especially for loose stones, to distinguish it from other gemstones.
Pleochroism
Beyond the color change, alexandrite exhibits strong pleochroism, meaning it shows different colors when viewed from different crystallographic directions. This can be observed by rotating the stone.
Inclusions
Examine for characteristic inclusions under magnification. While not always definitive, certain inclusion patterns can help identify natural alexandrite.
Similar Rocks
Chrysoberyl
BeAl2O4
Also known as: Cat's Eye (Cymophane)
Garnet (Color-change)
Pyrope-Spessartine series
Also known as: Color-change Garnet
Sapphire (Color-change)
Corundum var. Sapphire
Also known as: Color-change Sapphire
Scientific Classification
- Mineral Class
- Oxides
- Group
- Chrysoberyl Group
- Crystal System
- Orthorhombic
- Chemical Formula
- BeAl2O4
- Composition
- Beryllium aluminum oxide, with chromium (Cr3+) substituting for aluminum, causing the color change. Iron (Fe3+) can also be present.
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