Andalusite Value, Price, and Jewelry Information
Strongly pleochroic, andalusite can show shades of green, brown, and red when viewed from different directions. Although tough enough for most jewelry uses, this strikingly beautiful stone is largely unknown to the gem buying public.
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|Crystallography||Orthorhombic. Crystals prismatic, striated, square in cross section. Massive, compact.|
|Colors||Pinkish, reddish-brown, rose-red, whitish, grayish, yellowish, violet, greenish, colorless.|
|Luster||Vitreous to subvitreous.|
|Fracture||Even to subconchoidal|
|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).|
|Special Care Instructions||None|
|Transparency||Transparent to opaque.|
|Absorption Spectrum||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.|
|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 Andalusia, Spain (but see “Sources” below). 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.” Viridine alludes to the viridescent or “greenish” color of this variety.|
|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||Veils and rutile needles (common); liquid inclusions, hematite flakes (Brazilian material); carbonaceous inclusions (chiastolite).|
Andalusite is polymorphous with kyanite and sillimanite. This means these gemstones share the same chemistry but have different crystal habits. Although andalusite occurs abundantly as a mineral, material transparent enough to facet occurs quite rarely.
Andalusite has very distinctive and attractive pleochroism. This gem can show up to three different colors (trichroism) depending on the viewing angle and the gem’s cut and orientation. Sometimes, andalusites can even show multiple colors from one viewing angle. Gem shapes with a long axis such as ovals, marquise, 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.
Poorly cut and polished stones may appear dull. As Type II clarity grade gems, andalusites usually contain inclusions. However, a large, eye clean, well-cut andalusite demands attention. With a hardness of 6.5 to 7.5, it could find a home in almost any jewelry setting.
This grass to deep green variety of andalusite contains manganese.
Andalusites from Ottré, Belgium have a blue color due to an Fe+2-Fe+3 charge transfer mechanism.
Chiastolites are gray crystals with black, carbonaceous inclusions in cruciform patterns in their interior. Due to these impurities, chiastolite has a lower hardness and specific gravity (SG) than other andalusite varieties. Since it’s opaque, this variety is cut more or less as a curiosity. Typically, cross sections of chiastolite may show well-formed black crosses on a gray background. However, occasionally lapidaries cut these specimens into different shapes, such as spheres.
With its trichroism and red, green, and brown colors, andalusite may seem easy to distinguish from other gemstones. However, tourmalines have a similar hardness, color range, SG range, and moderate to strong pleochroism as well. Too casual observations of pleochroic colors can lead to confusion between these two different species of gems. (On the other hand, andalusites have a biaxial optic character, while tourmalines have a uniaxial optic character).
In fact, clever cutting can mimic andalusite’s strong pleochroism. For example, a predominantly brown synthetic quartz piece with a shallow green layer near its periphery was cut to show off both the colors, as real andalusites might. Again, more careful observation will distinguish the real item from a simulant.
Scientists have synthesized andalusite crystals via the hydrothermal method for research purposes. However, there is no known jewelry use for this material. If this gem becomes more well known, perhaps that will change.
Andalusites may rarely receive heat treatments to improve color.
Andalusite is eponymous with Andalusia, a region in southern Spain. This gem received this name from the French mineralogist Jean-Claude Delamétherie in 1798, after its supposed source. However, the material he examined came from Castile-La Mancha, a region squarely in the center of Spain. In fact, andalusite’s chiastolite variety had already been described by the Spaniard José Turrubia in 1754. Nevertheless, the name “andalusite” stuck, and so did references to Andalusia as the type locality. Although Andalusia does produce colorless andalusites, the type locality should be noted as El Cardoso de la Sierra, Guadalajara, Castile-La Mancha, Spain.
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.
Other notable gem sources include the following:
- 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; Austria; Madagascar; Russia; Spain; Zimbabwe.
Gems from Brazil can reach 75-100 carats. However, gems from most localities 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. Those 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).
Before subjecting any rare andalusites to mechanical cleaning systems, have a gemologist examine them and identify any inclusions they may have. While andalusites have a “Very Good” wearability score, inclusions of liquids or other minerals could shatter if heated or vibrated, thus damaging your gems. In the meantime, to be safe, use a soft brush, mild detergent, and warm water instead. Consult our gemstone jewelry cleaning guide for more recommendations.