Andalusite: The Complete Guide

Andalusite is a collector gem that deserves a much wider retail audience. Its extraordinary pleochroism — showing strong golden-brown, olive-green, and reddish-brown simultaneously in a well-cut stone, creating a shifting multicolour effect unlike any other gem — is one of the most distinctive optical effects in the mineral kingdom. Combined with excellent hardness, no cleavage, and beautiful colour character, andalusite is a gem that rewards the professional willing to champion unusual, high-quality alternatives.

What Andalusite Is

Andalusite is an aluminium silicate mineral (Al2SiO5), a polymorph of kyanite and sillimanite (same composition, different crystal structures). It belongs to the orthorhombic crystal system and has a Mohs hardness of 7-7.5 with good toughness and no significant cleavage — making it more durable than many better-known gems. The name derives from Andalusia, Spain, where the mineral was originally described, though fine gem-quality material comes primarily from Brazil.

The colour of andalusite ranges from brownish-red to greenish-yellow to olive in different crystallographic directions. The combination of these pleochroic colours in a single well-cut stone creates a visual effect that shifts from warm golden-green to reddish-brown as the stone moves under light — a multicolour effect somewhat reminiscent of alexandrite but with earthy rather than vivid spectral tones.

The Pleochroism Showcase

Andalusite is strongly pleochroic (trichroic), showing three distinct colours in its three crystallographic directions: reddish-brown (a-axis), olive-green to golden (b-axis), and near-colourless to pale yellow (c-axis). In a well-cut stone — typically a round or cushion with the table oriented to show the best combination — the viewer sees different combinations of these colours depending on the viewing angle, creating a dynamic, shifting multicolour that seems to change as the stone is tilted or moved.

This pleochroism is the defining quality factor for andalusite. The strength of the colour shift, the attractiveness of the combined colours face-up, and the evenness of the effect across the stone are the primary assessment criteria. Fine Brazilian andalusite from the Minas Gerais pegmatites shows some of the strongest and most attractive pleochroism in the species.

Sources and Commercial Availability

Brazil is the primary commercial source of gem-quality andalusite, particularly from pegmatites in Minas Gerais. Sri Lanka and Brazil also produce chiastolite, the variety of andalusite containing a cross-shaped dark inclusion of carbonaceous material visible in cross-section — historically called the “cross stone” and used as a religious talisman. Spain (the original locality) and a few other sources produce smaller quantities.

Commercial availability of fine andalusite is limited compared to mainstream gems. It is not available in every wholesale market and requires some sourcing effort. This scarcity at retail level, combined with its genuine optical interest, positions it as an ideal gem for professional jewellers seeking distinctive, differentiating inventory that cannot be found at every competitor.