Andalusite
Andalusite is a gemstone named after the Andalusia region of Spain, characterized by its remarkably strong pleochroism. Popular among gemstone enthusiasts, a single cut stone can display both greenish and reddish tones simultaneously, with the colors shifting depending on the viewing angle.
Andalusite, noted for its strong pleochroism
Basic Information
| English Name | Andalusite |
|---|---|
| Japanese Name | Kochuseki (Red Column Stone) |
| Chemical Formula | Al2SiO5 |
| System | Orthorhombic |
| Hardness | 6.5–7.5 |
| Specific Gravity | Approx. 3.15 |
| Transparency | Transparent to translucent |
Characteristics & Varieties
- Pleochroism: Strong pleochroism that appears to blend green and red depending on the orientation of the crystal.
- Chiastolite: Found in contact metamorphic rock (hornfels), characterized by a cross-shaped carbonaceous inclusion in the cross-section, forming an hourglass structure.
- Viridine: A variety that exhibits a vivid green color due to a high manganese content.
- Andalusite Cat's Eye: Chatoyancy rarely seen in brownish stones.
Origins, Aliases, Treatments, & Similar Stones
Primary Origins: Spain, Brazil, Sri Lanka, Madagascar.
Aliases: Brazilian Alexandrite, False Alexandrite (used for pleochroic Andalusite from Brazil).
Treatments: Impregnation (filling fractures in stones with synthetic resin to make them less visible).
Similar Stones: Dravite Tourmaline, Diopside, Epidote, Imperial Topaz
Note: Black and vivid green varieties are known as rare colors. Regarding Chiastolite, specimens with a cross intersecting at a right angle are considered more valuable than those with slanted intersections.