Andalusite

Andalusite · also called Chiastolite, Viridine, Cross stone

Andalusite is an aluminium silicate, Al₂SiO₅, cut as a gem for its exceptionally strong pleochroism, which shows green from one direction and reddish brown from another in the same stone. That is a directional effect, not a colour change. Chiastolite is its opaque variety, carrying a dark cross in cross-section.

What is andalusite?

Andalusite is an aluminium silicate, Al₂SiO₅, that crystallises in metamorphic rocks and is faceted as a gem for one property above all others: an exceptionally strong pleochroism that shows green from one direction and reddish brown from another within the same stone.

The mineral is one of three natural polymorphs of Al₂SiO₅, the others being kyanite and sillimanite. All three have the same composition and differ only in the pressure and temperature at which they form; andalusite is the low-pressure member, which is why it turns up in contact aureoles around intrusions and in low-pressure regional metamorphic belts rather than in deeply buried rocks. Most of the world's andalusite is never cut at all. It is mined in bulk as a refractory raw material, and gem-quality transparent crystals are a small by-product of a much larger industrial trade.

What gives andalusite its colour?

Iron is the principal colouring element, with manganese responsible for the green of the variety called viridine. The body colour is usually a brownish green to yellowish green, but what the eye registers is not one colour at all: it is the sum of two very different absorption directions seen at once.

Because andalusite is orthorhombic and strongly anisotropic, light travelling along different crystallographic axes is absorbed differently. One direction transmits a yellowish to brownish green, another a distinctly reddish brown to orange-red. In a cut stone the two arrive at the eye together and blend or alternate as the stone is tilted, which is the whole visual proposition of the material. Stones in which the reddish component is strong and clean, rather than muddy, are the ones that command interest.

How is andalusite identified?

The constants are narrow and diagnostic. Refractive index falls between 1.629 and 1.650, birefringence is low at 0.007 to 0.013, the optic character is biaxial negative, and specific gravity sits between 3.13 and 3.21. Dispersion is 0.016, moderate rather than showy.

Those figures do most of the separating on their own. Chrysoberyl, the stone andalusite is most often compared with, reads far higher at 1.746 to 1.755 with a specific gravity near 3.73. Tourmaline is uniaxial with a birefringence of about 0.018, high enough that facet edges double visibly through a loupe, and reads close to 1.62 to 1.64. Sphene has a dispersion and birefringence that no one would mistake for andalusite's. Smoky quartz and brownish zircon are eliminated at the refractometer in a single reading. A polariscope confirms the biaxial figure, and the pleochroscope confirms the rest.

The distinguishing feature: pleochroism, not colour change

Andalusite is frequently sold as a colour-change stone or as an inexpensive stand-in for alexandrite. It is neither. What it shows is pleochroism, which is a directional effect: the colours depend on the angle you look through the stone, not on the light source illuminating it.

The distinction is easy to test and worth insisting on. Hold the stone still and change the light from daylight to incandescent; a genuine colour-change stone such as alexandrite shifts, and andalusite does not. Hold the light constant and rock the stone instead, and andalusite shifts strongly while a colour-change stone does not. The effect is strong enough in good material that a single face-up view can carry green and orange-red simultaneously in different parts of the table, which is a look no other affordable gem produces in quite the same way. Cutters orient the rough deliberately to place both directions in the face-up view, and that orientation decision is the difference between a lively stone and a flat brown one.

What is chiastolite?

Chiastolite is the opaque variety of andalusite in which carbonaceous impurities are concentrated along the growing crystal's edges, so that a slice cut across the prism shows a dark cross set in a paler ground. It is cut as cabochons and slices rather than faceted.

The pattern is a growth phenomenon rather than an inclusion in the ordinary sense. As the crystal grows in a carbon-bearing pelitic rock, graphitic material is swept ahead of the advancing faces and accumulates preferentially at the corners of the square prism, producing four dark arms in cross-section. Where the growth rate changed, the arms change width along the length of the crystal, so no two slices are identical. The variety has a long history as an amulet material, and such uses are recorded as claims made by others rather than as properties of the stone.

Is andalusite treated?

Gem andalusite is generally untreated. There is no established heat or irradiation treatment that improves its colour, and because the material is inexpensive there is little commercial incentive to develop one. What reaches the trade is normally the stone as it came out of the ground, cut.

The caveats are about substitution rather than enhancement. Fracture-filling of surface-reaching breaks is possible in principle in any cut stone and should be looked for with reflected light and a loupe. Assembled stones and glass imitations exist at the low end of the market, though they are separated at once by refractive index. The most common misrepresentation is not a treatment at all but a name: material sold as colour-change andalusite, or as andalusite when it is in fact brownish tourmaline or chrysoberyl.

Where does andalusite come from?

Brazil supplies most of the faceted gem material, particularly from Minas Gerais and Espírito Santo, where it occurs as waterworn pebbles in alluvial gravels. Sri Lanka and Madagascar are the other consistent sources of transparent rough, with smaller quantities from Myanmar, Russia and the United States.

Spain retains the name but not the trade. Chiastolite has long been collected in the Iberian peninsula, and the variety is also produced in China, in California and at Lancaster in Massachusetts, where the crossed slices have been sold for generations. Bulk andalusite for refractory use comes overwhelmingly from South Africa and from Western Australia, mined at a scale that dwarfs the gem trade but yields almost nothing transparent. Country of origin carries no premium in andalusite: the market prices the stone on what it looks like, and a Brazilian and a Sri Lankan stone of equal quality sell for the same money.

Where the name comes from

The mineral was described at the end of the eighteenth century and named for Andalusia in southern Spain, the region given as the source of the material examined. Later authors concluded that the original specimens more probably came from El Cardoso, in Guadalajara province, well outside Andalusia.

The name stuck regardless, which is common enough in mineralogy. Chiastolite takes its name from the Greek letter chi, written as a cross, in reference to the figure seen in a transverse slice. Viridine, the manganese-rich green variety, takes its name from the Latin for green. Andalusite has also been marketed under invented trade names implying a relationship to alexandrite, and those names should be treated as sales language rather than description.

What sizes are available?

Clean faceted andalusite is common below three carats, becomes noticeably harder to find above five, and is genuinely scarce above ten. The rough is not rare, but transparent crystals of any size are, and the strong pleochroism forces the cutter to orient the stone for colour rather than for weight.

That orientation constraint is the real limit on yield. A cutter who takes the largest possible stone from a piece of rough will often lose the reddish direction and finish with something flat and brown, so material is routinely sacrificed to keep the play of colour in the face-up view. Elongated shapes, ovals and cushions dominate for that reason. In larger sizes the pleochroism tends to read as patchiness rather than as an even blend, so bigger is not automatically better, and the most attractive stones are often in the two to five carat range.

How much is andalusite worth?

Andalusite is an inexpensive gem, and almost all of the price variation comes from colour quality rather than from rarity. The factors in order of weight are the strength and cleanliness of the pleochroic colours, the absence of a muddy brown cast, clarity, cut orientation and precision, and only then size.

A stone in which the green is a true green and the reddish direction a clear orange-red is worth a substantial multiple of one in which both directions have collapsed towards the same dull brown, and that difference is visible across a table without any instrument. Clarity should be high; the material is available clean, so visible inclusions are a reason to pass rather than to negotiate. Cut matters more than usual because orientation is a judgement call the cutter cannot take back. Size adds value only once the colour is already good.

Is andalusite durable enough to wear?

Andalusite is 7 to 7.5 on the Mohs scale, which is hard enough for everyday jewellery including rings. The reservation is cleavage: the mineral has a distinct prismatic cleavage, so it is less forgiving of a sharp knock than a stone of similar hardness with none.

In practice that means protective settings for ring stones and a degree of care in setting, since pressure applied along the wrong direction can start a cleavage plane. Fracture is uneven to subconchoidal. Warm water, mild soap and a soft brush are the sensible cleaning method. Ultrasonic cleaners are best avoided on a stone with cleavage, particularly if any inclusion reaches the surface, and steam cleaning is unnecessary risk for no gain.

How to buy andalusite without being caught out

Test the colour claim before anything else. Keep the light constant and rock the stone: if the colours move, that is pleochroism working as it should. Then keep the stone still and change the light source; if nothing happens, the stone is not a colour-change gem, and any claim otherwise is wrong.

Look at the face-up view in daylight rather than under warm shop lighting, which flatters the orange direction and hides brown. Both pleochroic colours should be clean; a stone where everything has slid towards mud is the common failure and no amount of size compensates. A refractometer reading of 1.629 to 1.650 with a biaxial figure confirms the species and rules out tourmaline, chrysoberyl and quartz at a stroke. Ask whether the stone is natural and untreated, and expect the answer to be yes. Judge the cut hard, since orientation is where the value sits, and be sceptical of any origin premium. If you are looking for a particular size or orientation, tell us what you are after or see what is currently listed in our gems.

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