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.
Start an IGS Membership today for full access to our price guide (updated monthly).
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°.
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.
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.
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.
What are the Best Gem Shapes for Andalusite Jewelry?
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, so a large, eye-clean, well-cut andalusite will demand attention. With a hardness of 6.5 to 7.5, it could find a home in almost any type of jewelry setting.
You may find chiastolites for sale as trapiche andalusites. These gray crystals have 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 trichroic 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).
Gems from Brazil can reach 75 to 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, and those over 20 carats are still rarer.
Royal Ontario Museum (Toronto, Canada): 12.44 (Brazil).
Caring for Your Andalusite Jewelry
Although resistant to scratching due to its hardness, andalusite has distinct cleavage and brittle tenacity. Therefore, ring stones should have protective settings to avoid blows.
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.