Andalusite

Andalusite varies in colour from pale yellowish brown to some dark bottle-green, dark brown, or the most popular greenish red. It has very strong and distinctive pleochroism, so that, when turned, the same stone may appear yellow, green and red. Large crystals might be vertically striated prisms having a square cross-section and pyramidal ends, but they are rare. More usual are opaque, rod-like aggregates of crystals or waterworn pebbles. It is the pebbles which are usually cut as gemstones.

Andalusite is generally found in pegmatites. Pebbles occur within the gem gravels of Sri Lanka and Brazil. Other localities include Spain, Canada, Russia, Australia and the United States.

An opaque, yellowish grey variety, chiastolite, occurs as long prisms, which make a cross when cut and polished.

Andalusite Properties

Chemical Composition: 
Aluminium silicate with trace elements of iron or manganese. Al2SiO5
Classification / Type: 
Polymorphous with Kyanite and Sillimanite.
Colors / Varieties: 
  • Brown, green, yellow common. Rarely seen in pink and violet. (Green known as viridine)
  • Chiastolite is an opaque variety of andalusite which contains a black cross-like core made up of carbonaceous material in a grey - brown background.
Crystal System / Forms: 
Orthorhombic System / Vertically striated prisms capped with pyramids; much of the gem material as water-worn pebbles.
Hardness: 
7 - 7.5
Specific Gravity: 
3.18 ± 0.04
Cleavage / Fracture: 
Easy prismatic cleavage / Conchoidal fracture.
Optic Character: 
Anisotropic, D.R.; Biaxial negative.
Lustre: 
Vitreous.
Refractive Index / Birefringence: 
1.634 - 1.643 / 0.008 - 0.013 (birefringence is highest when the indices are lowest). Range: 1.629 - 1.664
Pleochroism: 
Very strong. Generally light yellow green and dark brownish red and brownish green.
Magnification: 
Rutile needles, fingerprints, crystals. Chiastolite exhibits a dark-cross.
U.V. Fluorescence: 
May fluoresce dark green to yellow green under shortwave.
Spectrum: 
  • Manganese spectrum: seen at 553.5nm and fine lines at 550.5nm and 547.5nm, band at 455nm.
  • Rare earth spectrum at 580nm in some yellow, green andalusite.
Cause of Color: 
  • Green (viridine) andalusite: manganese
  • Brownish Green andalusite: charge transfer
Specific Tests: 
The strong pleochroism is often mistaken for a color change effect.
Simulants (with separation tests): 
Tourmaline (optic figure, birefringence, pleochroism), Epidote (R.I., S.G.), Alexandrite (R.I., S.G., pleochroism, inclusions), Apatite (optic figure, pleochroism).
Geological Occurrence: 
In metamorphic rocks, within mica schist or gneiss
Sources: 
Sri Lanka, Spain, Brazil, Myanmar, Russia (Ural Mountains), U.S.A. (Arizona, California).
Cuts & Uses: 
Facetted cuts, cabochons, beads. Generally in fancy cuts to bring out its strong pleochroic property.