Chiastolite is a variety of andalusite characterized by unique, cross-shaped dark inclusions of carbonaceous material or clay. It is primarily sought by collectors for its natural cruciform patterns and is commonly found in low-grade regional metamorphic rocks.
Is this chiastolite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch chiastolite with a known reference. Chiastolite sits at Mohs 6.5-7.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: white.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: gray, yellowish-brown, reddish-brown.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: prismatic crystals with a distinct black carbon cross inclusion.
Often confused with
Chiastolite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside chiastolite
Minerals reported to co-occur with chiastolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Al₂SiO₅
- Mohs hardness
- 6.5-7.5
- Density
- 3.1-3.2 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Prismatic crystals with a distinct black carbon cross inclusion
- Cleavage
- Good in two directions
- Rarity
- Common
- Uses
- Collector, Lapidary, Jewelry
- Host rock
- metamorphic rocks like schist and slate
Where rockhounds find chiastolite
1 mapped spotsClassic worldwide localities
- Spain
- Australia
- USA
- China
- France
Field-hunting tip
Where it forms: metamorphic rocks like schist and slate. If you start seeing quartz, mica, garnet in float, you are in the right ground. Typical habit: prismatic crystals with a distinct black carbon cross inclusion. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Massachusetts. Start trip planning there.
Common questions
How do you identify chiastolite?+
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Can I find chiastolite in the United States?+
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Find chiastolite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
