silicates
Andalusite
Al2SiO5
- Also known as
- chiastolitecross stonemacle
- Crystal system
- orthorhombic
- Hardness
- 6.5–7.5 on the Mohs scale
- Lustre
- vitreous
- Colour
- Brownish green to reddish brown, strongly changing with viewing angle
- Where it forms
- In metamorphic rocks at low pressure, the low-pressure counterpart of kyanite.
- Chiefly from
- Andalusia, SpainBrazilSri LankaAustria
Why it is that colour
Iron and manganese. Its striking feature is pleochroism — it shows quite different colours along different crystal axes, so a cut stone flashes green and red as it turns.
Safety
Safe to handle.
Where it comes from
Mined for refractories. Chiastolite comes chiefly from Spain, China and California.
Historical use
Chiastolite crosses were sold at Compostela and elsewhere as pilgrim tokens. The cross is formed by graphite inclusions concentrated along the crystal’s growth axes, so it runs the whole length of the crystal and appears in every slice.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
The chiastolite variety, which contains a natural dark cross in cross-section, was used as a Christian amulet in medieval Europe and in Spain particularly, where pilgrims carried it. That use is documented. Andalusite generally has no separate tradition.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Chiastolite is a good case of a mineral producing something that looks designed and is not. As an andalusite crystal grows, carbon impurities are pushed ahead of the advancing faces and concentrate at the corners, so the finished crystal has dark lines running its whole length along four edges. Slice it across and you get a cross, every time, all the way through.
Medieval pilgrims took this as a sign. It is a straightforward consequence of how impurities behave at a growing crystal face — and it is genuinely remarkable that geometry alone produces something so specific.
Identity checked against IMA-CNMNC master list, June 2025.