Andalusite Identification Guide
Identify andalusite by its strong pleochroism, near-square prisms, and the chiastolite cross variety, and separate it from kyanite and sillimanite.
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What Andalusite Looks Like
Andalusite is an aluminum silicate (Al2SiO3, polymorphic with kyanite and sillimanite) that crystallizes in the orthorhombic system. Crystals are stubby prisms with a nearly square cross-section, typically brown, reddish-brown, pinkish, olive-green, or gray. Luster is vitreous, transparency ranges from transparent gem material to opaque. Its standout optical feature is strong pleochroism: a gem-quality stone visibly shifts among greenish, brownish-red, and yellow as you rotate it.
The famous variety chiastolite contains dark carbonaceous inclusions arranged in a cross or checkerboard pattern seen in cross-section — a unique field marker.
Step-by-Step Field-ID Checklist
- Look for square or rectangular prism cross-sections in matrix.
- Rotate a transparent piece in light — strong color change (pleochroism) between green and reddish-brown is highly diagnostic.
- In gray/brown opaque prisms, cut or check the cross-section for the dark chiastolite cross.
- Scratch test — Mohs 7–7.5; it scratches glass and quartz.
- Examine the host rock — andalusite favors metamorphosed shales (hornfels, schist).
Key Diagnostic Tests
- Mohs hardness: 7–7.5.
- Streak: white.
- Cleavage: good prismatic cleavage in two directions.
- Specific gravity: ~3.1–3.2.
- Strong pleochroism under rotation — a key separator from look-alikes.
- Inert to acid, non-magnetic.
Common Look-Alikes
- Kyanite (same composition): much softer along its length (Mohs ~4.5–5) and harder across (~6–7), typically blue and bladed; andalusite has uniform hardness ~7.5 and squarer prisms.
- Sillimanite (same composition): fibrous or needle-like habit ("fibrolite"), whereas andalusite is stubby and square-sectioned.
- Smoky quartz / tourmaline (brown gems): quartz lacks andalusite's strong green-red pleochroism; tourmaline shows rounded-triangular cross-sections and stronger striations.
- Topaz: has perfect basal cleavage and higher density; weaker pleochroism than andalusite.
Where It Is Found
Andalusite forms in low-pressure, contact-to-regional metamorphosed aluminous rocks — hornfels and schists around granite intrusions. Notable localities include Andalusia, Spain (the namesake region), Brazil and Sri Lanka (gem pebbles), Austria, Russia, and the U.S. (California, Massachusetts for chiastolite). Chiastolite "cross stones" come famously from California and Spain.
Frequently asked questions
How can you tell if it's real andalusite?
Genuine andalusite is Mohs 7–7.5, has near-square prismatic crystals, and shows strong pleochroism (visible green-to-reddish-brown color shift as you rotate it). The chiastolite variety shows a dark internal cross in cross-section.
What does andalusite look like?
It appears as stubby brown, reddish, pink, or greenish prisms with a roughly square cross-section and a glassy luster; gem stones flash different colors when turned, and chiastolite shows a built-in dark cross.
Andalusite vs kyanite — what's the difference?
They share a chemical formula but differ physically: kyanite is bladed, usually blue, and has very different hardness in two directions (about 4.5 along, 6.5 across), while andalusite is square-sectioned with a uniform hardness near 7.5.
What is chiastolite?
Chiastolite is a variety of andalusite with dark graphite inclusions arranged in a cross or checkerboard pattern, visible when the prism is cut across its length. It is popular as a natural cross-stone.
Andalusite identified by the community
Recent Andalusite specimens identified with Rock Identifier.
