Andalusite | Gemopedia

Andalusite, an aluminum silicate, derives its name from the southern Spanish province of Andalusia, long believed to be the sight of its original discovery. Many now feel the original find may have actually occurred in El Cardoso, a different province in Spain, but the name endured. While andalusite's color play has been compared to alexandrite, this is technically incorrect, as it is pleochroic rather than color changing. Prized for its strong pleochroism, it displays different colors in different directions and features all its colors at once, whereas true color-change gems like alexandrite only change color when exposed to different light sources.
Alternate
Names
Viridine, Chiastolite
Colors
Yellowish Green To Brownish Green, Orangy Brown

Andalusite Classification

Common Name

Andalusite

Species

Andalusite

Variety

Colors

Yellowish Green To Brownish Green, Orangy Brown

Alternate Names

Viridine, Chiastolite

Gemstone Groups

Key Separations

Pleochroism is usually evident from general observation. Identity may be confirmed by refractive index, birefringence, and optic character.

Comments

Andalusite is a polymorph of Al2SiO5 along with kyanite and sillimanite. Misnomers: poor man's alexandrite. Viridine is the dark green variety containing iron and manganese. Chiastolite is translucent to opaque and displays a cross.

Andalusite Optical Properties

Transparency

Transparent - Opaque

Refractive Index

1.634-1.643
Tolerance:(+0.005/-0.005)

Birefringence

0.007-0.013

Optic Character

Biaxial

Optic Sign

Negative

Polariscope Reaction

Doubly Refractive (DR)

Fluorescence

SWUV: Inert to moderate green to yellowish green
LWUV: Inert

CCF Reaction

Pleochroism

Trichroic, strong yellowish green, green and brownish red

Dispersion

Strength: weak fire Value: 0.016

Comments

Andalusite Chemistry & Crystallography

Chemical Name

aluminum silicate

Chemical Formula

Al2SiO5

Synthesis

Crystal System

Orthorhombic

Classification

Silicate

Nature

Natural

Crystallinity

Crystalline

Comments

Andalusite Characteristic Physical Properties

Hardness

7.5

Streak

White

Specific Gravity

Typical:3.17

Toughness

Varies

Inclusions

Andalusite is a type II clarity stone. The needles or crystals seen in the stone are most likely rutile and apatite.

Luster

Vitreous

Stability

Good

Fracture

Uneven

Cleavage

Good, in one direction, Poor, in one direction

Comments

Andalusite
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