Andalusite is a strongly pleochroic gemstone, which means it can show different colors when viewed from different directions. Although andalusite gems are hard and tough enough for most jewelry uses, this strikingly beautiful gem is largely unknown to the gem buying public. Depending on the cut and orientation, these stones can show shades of brown, green, and reddish brown.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
|Crystallography||Orthorhombic. Crystals prismatic, striated, square in cross section. Massive, compact.|
|Refractive Index||1.629 - 1.650|
|Colors||Pinkish, reddish-brown, rose-red, whitish, grayish, yellowish, violet, greenish.|
|Luster||Vitreous to subvitreous.|
|Fracture||Even to subconchoidal. Brittle.|
|Hardness||6.5 - 7.5|
|Specific Gravity||3.13 - 3.17|
|Birefringence||0.007-0.011. (Viridine: 0.029.)|
|Cleavage||Distinct 1 direction.|
|Luminescence||None in LW. Brown fluorescence in SW (Lancaster, Massachusetts). Dark green or yellow-green fluorescence in SW (brown-green gems from Brazil).|
|Spectral||Deep green varieties from Brazil display Mn spectrum: knife-edge shadow at 5535, fine lines at 5505 and 5475; faint lines at 5180, 4950, and 4550.|
|Enhancements||Can be heat treated to improve color. Rarely done.|
|Special Care Instructions||None|
|Transparency||Transparent to opaque.|
|Phenomena||Chatoyancy (cat's eye), very rare.|
|Formula||Al2SiO5 + Fe|
|Pleochroism||Strongly pleochroic; olive green to flesh-red (Brazil). Usually yellow/green/red. Blue andalusite from Belgium: blue/colorless/colorless.|
|Optics||α = 1.629 – 1.640; β = 1.633 – 1.644 γ = 1.638-1.650. Near-colorless andalusite reported at low end of this range; green material at upper end. Viridine: 1.66-1.69. Biaxial (-), 2V = 73 - 86°.|
|Etymology||After the first noted locality, Andalusia (Spain). Chiastolite is from the Greek chiastos, “arranged diagonally,” because the pattern of carbon inclusions in the gem resembles the Greek letter chi, which is written “X.”|
|Occurrence||Metamorphic rocks, usually slates and schists as a contact mineral, or developed within mica schist or gneiss. Also as a detrital mineral and very rarely in pegmatites and granites.|
|Inclusions||Rutile needles, common. Liquid inclusions.|
Andalusite’s pleochroism is very distinctive and attractive. Andalusite can show up to three colors (trichroism). Gem shapes with a long axis such as ovals, marquis, or emerald cuts tend to show one color near the center and a second, usually darker color, near the ends. Square and round cuts usually blend the colors into a mosaic. Sometimes, lapidaries cut these stones to show pink and almost colorless shades. Others cut to display green in the center of these stones, with browns or various other combinations on the tips, depending on the rough orientation before cutting.
Although poorly cut and polished stones may appear dull, a large, clean, well-cut andalusite demands attention in any jewelry setting.
Viridine, a deep green variety of andalusite, contains manganese.
The blue color in andalusites from Ottré, Belgium is due to an Fe+2-Fe+3 charge transfer mechanism.
Since it’s opaque, chiastolite is cut more or less as a curiosity. Cross sections of this material may show a well-formed black cross on a gray background. Due to the impurities it contains, chiastolite has a lower hardness and density than other andalusite varieties.
Currently, Brazil is the main source of these gems. They can be found as pebbles in stream beds or on hillsides under layers of clay. Andalusia, Spain, the stone’s namesake, produces a colorless variety.
Other notable gem sources include:
- United States: California; Colorado; Maine; Massachusetts; New Mexico; Pennsylvania; South Dakota (Black Hills).
- Belgium: blue crystals.
- Myanmar: dull green material found in gem gravels.
- Sri Lanka: gem material found as waterworn pebbles, sometimes large size.
- Australia; East Africa; Madagascar; Russia.
Gems from Brazil reach 75-100 carats. Gems usually range from 1 to 5 carats. Andalusites in the 5 to 10-carat range cost several times more per carat than smaller stones. Stones over 10 carats are quite rare. Stones over 20 carats are still rarer.
- Smithsonian Institution (Washington, DC): 28.3 (brown, Brazil), 13.5 (green/brown, Brazil).
- Royal Ontario Museum (Toronto, Canada): 12.44 (Brazil).
Although resistant to scratching due to its hardness, andalusite is slightly brittle due to its cleavage. Ring stones should have protective settings to avoid blows. Consult our gemstone jewelry cleaning guide for more recommendations.