Andalusite stones or andalusite is an aluminium neso-silicate mineral with the chemical formula Al2SiO5. Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temperatures and pressures, andalusite may convert to sillimanite. Thus, as with its other poly-morphs, andalusite is an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.
Andalusite is sometimes called “The Seeing Stone” because it is used in metaphysical works to calmly see the various parts of one’s character without bias. It is also reported to be helpful to see the different sides of a problem, and is used for scrying.
Andalusite is named after Andalusia, the Spanish autonomous community where it was first discovered. Andalusite is an aluminum silicate, closely related to both silimanite and kyanite. In fact, all three minerals are polymorphs, which means they share the same chemical composition, but possess different crystal structures. Andalusite is a strikingly beautiful gem, but it is largely unknown to the general public and considered to be one of the lesser-known gem types in the trade.
Andalusite stones have a very distinct combination of colours, and a very pronounced level of pleochroism, which results in the exhibition of different colors when viewed from different angles. Andalusite most often occurs translucent to opaque, with transparent gemstone-quality specimens being very rare. For many years, andalusite has primarily been a collector’s stone, but it has recently gained a lot of attention from many jewelry designers. It is becoming increasingly popular in jewelry designs. Andalusite possesses a good level of durability and hardness, making it suitable for any type of jewelry application. The attraction of andalusite is greatly owed to its play of color, which can be seen during changes in its viewing angle. Similar effects are also seen when lighting strikes the gem from different directions.
There are only a few gem types that could be mistaken for andalusite, including tourmaline, chrysoberyl, sphene, smoky quartz and idocrase. Pleochroism in gems occurs in varying strengths; weak, distinct or strong. Pleochroic effects are the result of differing absorption of light rays, and the phenomenon can only occur with doubly refractive crystals. Andalusite is considered to be strongly pleochroic, along with iolite, kyanite, kunzite, sphene and tanzanite. Andalusite has trichroic pleochroism; when light enters the stone, it is parted into three sections, each containing a portion of the visible spectrum. Pleochroic gems, such as kunzite, possess dichronic pleochroism, which means that they display only two different colours.
Andalusite stones typically occurs in placers, gneisses, and schists as a result of argillaceous sediment that has been metamorphosed. Andalusite rarely occurs in granite or pegmatites, but it does, it tends to yield the largest crystals. Andalusite deposits can be found in many locations, including Australia, Brazil, Canada, Russia, Spain (Andalusia), Sri Lanka, Burma, Madagascar and the USA (California and Colorado).
Andalusite stones colours depend on the orientation of the crystal, but they typically occur yellow, yellow-green, green, brownish red, olive and reddish brown. Each gemstone possesses two colours that differ in intensity, and often the colours blend together, especially with square and round shapes. Shapes with a long axis, such as oval, pear, marquise or emerald cuts, tend to show one colour near the center and a second, usually darker colour toward the ends of the crystal. Typically, when cutting pleochroic gems, cutters attempt to minimise pleochroism and maximise one desirable colour. However, with andalusite, cutters do the exact opposite and try to orient the gem to result in a pleasing mix of colours, such as orangey-brown and yellowish-green or gold.
Andalusite stones are typically occurs translucent to opaque. Opaque specimens are known as chiastolite. Dark inclusions in chiastolite produce cruciform-like shapes within the stones and these are often referred to as ‘Cross Stones’. Chiastolite usually occurs white, gray or yellowish and is rather soft compared to transparent andalusite. Transparent andalusite is quite rare and only a small percentage of yield is of gemstone quality. Most specimens contain some inclusions, with the most common being rutile needles. When polished, andalusite has a vitreous to matte luster.