Andalusite is one of three polymorphs of aluminium silicate (Al₂SiO₅), alongside kyanite and sillimanite, each crystallising under different conditions of temperature and pressure. Of the three, andalusite is the polymorph that produces the most consistently faceted gem material, and it is the one in which the optical phenomenon of pleochroism reaches its most striking expression. Despite a rich gemmological character — strong colour change with viewing angle, very good hardness and durability, and a charm independent of the more famous coloured stones — andalusite remains a connoisseur's gem. It does not appear in the bridal mainstream, and good cutting is rewarded with a colour play that few other species can match.

Mineralogy

Andalusite is named after the Andalusia region of Spain, the locality where it was first identified in the 1790s. The species crystallises in the orthorhombic system, with prismatic to columnar habit. Its hardness is 6.5 to 7.5 on the Mohs scale, specific gravity around 3.13 to 3.21, and refractive indices of approximately 1.633 to 1.644. Cleavage is good in two directions but not perfectly developed, and the species is reasonably tough. The combination of hardness above 7 and good toughness makes andalusite fully suitable for ring wear.

An iron-rich variety known as chiastolite develops dark inclusions of carbonaceous material that arrange themselves in a cross-shaped pattern visible in cut sections perpendicular to the c-axis. Chiastolite has a long history of folk use in religious jewellery, where the natural cross has been read as a Christian symbol. Faceted gem-quality andalusite, by contrast, is generally free of these inclusions and has the transparency of a fine gem.

The pleochroism

Andalusite's defining gem character is its strong pleochroism: the property by which a doubly-refractive crystal absorbs light differently along its different optical axes, displaying different colours when viewed from different directions. Andalusite is biaxial, and good gem material shows three distinct colours along its three optical directions: typically yellowish-green, brownish-red, and reddish-orange in iron-bearing material, sometimes with a smoky brown component. The skill of the cutter is to orient the table and pavilion of the finished stone so that the eye sees a balanced mosaic of the three colours simultaneously, rather than dominantly one. Properly cut andalusite presents a play of green, gold, and orange-red that is unique among species.

Sources

The most important historical source for facetable andalusite is Brazil, particularly Minas Gerais and Espírito Santo. Sri Lanka has produced fine material for centuries, alluvially associated with the gem gravels that yield the country's broader range of gem species. Significant production has also come from Madagascar, Tanzania, and Russia. Spain, the type locality, produces principally chiastolite specimens rather than facetable rough.

Cutting and grading

Cutting andalusite well requires understanding the orientation of the optical axes in the rough. A skilled cutter selects the orientation that produces the most attractive balance of colours in the finished stone, often sacrificing some weight to get the colour right. Stones are typically cut as ovals, cushions, and emerald cuts, where the elongated form aligns with the c-axis and presents the pleochroic colours in a pleasing distribution.

Quality grading for andalusite emphasises pleochroism strength, balance of the three colours in the face-up view, transparency, and absence of internal cloudiness. Stones with weak or muddy pleochroism, or with cloudy bodies that diffuse the play of colours, trade at significantly reduced prices.

Treatment

Andalusite is not commonly treated. Its colour is natural and stable. There are no widely used heating, diffusion, or fracture-filling treatments for the species, although some authorities note that low-temperature treatments occasionally improve poor pleochroism. Reputable laboratory reports typically state no indications of treatment for natural-colour stones.

In the trade

Andalusite is among the best values in the connoisseur coloured-stone trade. Fine 2-to-5-carat stones with strong, balanced pleochroism trade at modest prices relative to their visual interest. The stone rewards close looking and bench-light comparison, and clients who appreciate the subtleties of pleochroism find andalusite among the most rewarding species to examine. We commonly fit fine andalusite into rings, where its hardness allows confident wear, and into pendants where the play of colour is visible to the wearer's own eye.

Further reading

• GIA — Andalusite reference
• International Gem Society — Andalusite