Andalusite: several colours in one view

Look again at a clear andalusite. As it turns, you may see yellow-green, green and brownish-pink tones. This is pleochroism. Light takes different paths through the crystal and shows different colours. A cut stone can show several paths at once. Its cutting direction helps shape the effect. This does not promise a change of colour between different lamps. GIA explains how pleochroism affects faceted gems.

Evidence: GIA’s explanation of pleochroism

Identity, formation and sources

Andalusite has the formula Al₂SiO₅ and an orthorhombic crystal structure. Kyanite and sillimanite have the same chemistry. Their atoms form different structures. The separate species are called polymorphs. They are not colour varieties of one gem. Andalusite often forms when clay-rich sediments change through metamorphism. This includes rocks heated near igneous intrusions. It also occurs in schists changed by regional metamorphism. These links come from the Mineralogical Society of America’s entry.

Records list Spain, Austria, Germany, Australia and the United States. The records also name Brazil and Sri Lanka as sources of gem material. A mineral can occur widely without every place producing clear rough for cutting. The same MSA source notes gem crystals in its place records.

Chiastolite is a variety of andalusite with a dark cross-shaped pattern. Inclusions of carbonaceous matter make the cross. It is often opaque. A 2009 study found unusual clear examples. A chiastolite cross and a clear pleochroic gem belong to one species. They offer very different views.

Evidence: The Mineralogical Society of America’s entry, Read GIA’s Brazilian andalusite study

A scientific story beneath the sparkle

Andalusite helps us study the heat and pressure inside Earth. Its overall chemistry is the same as kyanite and sillimanite. Their structures differ. They show why pressure and temperature matter. A phase diagram brings test results together. It shows which conditions favour each structure. GIA uses this trio to explain gemstone deposits. Knowing the ingredients alone does not tell us which crystal will form.

Evidence: Explore the aluminosilicate example

Treatments, synthetics and care

A 2009 study heated andalusite samples. Colour changed little, or only subtly. Resin filling made the stones look clearer. These findings concern those samples. They cannot promise the same for each stone on sale. Ask which treatments were used. GIA’s summary supports that request. “Synthetic” and “imitation” have different meanings. A laboratory-grown counterpart has essentially the same chemistry and structure as the natural material. A lookalike may be another material. Colour alone cannot tell us what a gem is, or if it formed in nature.

MSA gives andalusite a Mohs hardness of 6.5–7.5. It also describes brittleness and cleavage. Protect the corners and edges from blows. If treatment history is unknown, our cautious advice is mild soap, lukewarm water and gentle hand-cleaning. Avoid heat, steam, ultrasound and solvents until a jeweller has checked its condition. GIA’s care advice explains the risks to filled stones.

Evidence: Read GIA’s Brazilian andalusite study, GIA explains synthetic gems, GIA’s jewellery-care advice

Two comparisons to keep separate

Describe two images of the same gem before naming an effect. Was the stone turned under the same light? Or was the light changed? Pleochroism asks about direction. A colour-change study asks about the light. Keep the two questions apart. Next, compare andalusite, kyanite and sillimanite. They share a formula but have different structures. Colour and ingredients are useful clues. They do not tell the whole story of what it is. The phase diagram gives a next question: which conditions favour each structure?

Evidence: GIA’s explanation of pleochroism, Explore the aluminosilicate example