Chlorastrolite is a rare, fibrous variety of pumpellyite prized by lapidary enthusiasts for its distinctive 'turtle-back' radial pattern. It is typically found as amygdules within basaltic rocks and is recognized as the official state gemstone of Michigan.
Is this chlorastrolite?
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 chlorastrolite with a known reference. Chlorastrolite sits at Mohs 5-6 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Chlorastrolite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Chlorastrolite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: green, bluish-green.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal, radial fibrous.
Often confused with
Chlorastrolite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside chlorastrolite
Minerals reported to co-occur with chlorastrolite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Ca₂MgAl₂(SiO₄)(Si₂O₇)(OH)₂·H₂O
- Mohs hardness
- 5-6
- Density
- 3.2 g/cm³
- Colors
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Botryoidal, Radial Fibrous
- Cleavage
- None
- Rarity
- Rare
- Uses
- Gemstone, Lapidary, Collector
- Host rock
- Basaltic Lava Flows
- Typical price
- $20-200 per gram depending on pattern quality
Where rockhounds find chlorastrolite
2 mapped spotsClassic worldwide localities
- Isle Royale, Michigan, USA
- Keweenaw Peninsula, Michigan, USA
Field-hunting tip
Look in basaltic lava flows country — that is the host setting where chlorastrolite typically forms. If you start seeing native copper, datolite, prehnite in float, you are in the right ground. Field specimens usually show a botryoidal, radial fibrous habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Michigan, Minnesota — start trip planning there.
Common questions
How do you identify chlorastrolite?+
Where is chlorastrolite found?+
Can I find chlorastrolite in the United States?+
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Find chlorastrolite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
