Andalusite

Andalusite is an aluminium silicate (Al₂SiO₅) mineral; it is one of three siblings with the same chemical formula but stable under different conditions, and the one that forms at low pressure. Thanks to its strong pleochroism, the same crystal looks reddish pink, green and yellow when viewed from different directions. The variety in which carbon or clay remains line up in the shape of a cross is called chiastolite. Its name points to the Andalusia region of Spain.

Ornamental stone: Clear andalusite, thanks to its strong pleochroism, shows a play of red, green and yellow when cut. Clean crystals from Brazil and Sri Lanka are cut into jewellery. Chiastolite: Cross-patterned crystals, sliced and polished, have been carried for centuries as keepsakes and charms. Industry: Andalusite is used as a refractory (heat-resistant) material in furnaces, kilns and other high-temperature processes. According to Wikipedia, about a quarter of world production comes from the Glomel mine in France, while South Africa holds the largest share of known deposits.

It has a hardness of 6.5–7.5 on the Mohs scale. It scratches glass and steel; a steel knife cannot scratch it. Its specific gravity is 3.13–3.17 g/cm³.

Its best-known localities: El Cardoso de la Sierra, Guadalajara, Spain; Glomel, Côtes-d'Armor, France; Lancaster, Massachusetts, USA; Santa Teresa, Espírito Santo, Brazil; Nawalapitiya, Central Province, Sri Lanka.

Known localities in Türkiye: Sivrihisar, Eskişehir, Türkiye.

Prismatic crystals with a nearly square cross-section; in the chiastolite variety, when the crystal is cut, dark carbon or clay inclusions form a cross or “X” pattern. Its hardness is 6.5–7.5: a steel knife cannot scratch it. It is strongly pleochroic; as you turn the stone its colour shifts between reddish pink and greenish yellow. It is found in schists formed by contact or regional metamorphism of clay-rich rocks, together with kyanite, sillimanite, cordierite, garnet and mica; it is rare in granites and pegmatites, but they yield the largest crystals.

On the “street to diamond” value ladder it sits on step 5 of nine: Ornamental stone. As an ornamental stone, the value of andalusite depends on colour and pleochroism (the play of red, green and yellow), on clarity and on the quality of the cut; clean, vividly coloured, large stones are rare, and those from well-known sources such as Brazil and Sri Lanka are more sought after. In chiastolite the sharpness of the pattern and the symmetry of the cross matter. In industry its value depends on the size of the deposit, grain size and purity: it is used in heat-resistant materials.

According to George F. Kunz (1913), chiastolite worn so as to touch the skin was believed to stop the flow of blood from any part of the body and to increase the secretion of milk, and worn hanging from the neck to cure all kinds of fever. According to the same source, because the cross on the stone was seen as a sacred symbol, chiastolite was believed to drive evil spirits away from its wearer, and the stone was regarded as an object of mystical and religious significance. There is no evidence that chiastolite stops bleeding, increases milk or lowers fever; for bleeding or high fever, medical help is needed, not a stone. The cross pattern is a wholly natural phenomenon, formed when carbon and clay remains line up in an orderly way as the crystal grows. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.