Isle Royale Greenstone, also known as Chlorastrolite, is a rare gemstone found in the basaltic lava flows of the Lake Superior region. It is highly prized by collectors for its distinctive 'turtleback' pattern, which consists of radiating dark green fibrous crystals trapped within a lighter green matrix.
Is this isle royale greenstone?
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 isle royale greenstone with a known reference. Isle Royale Greenstone 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. Isle Royale Greenstone leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Isle Royale Greenstone typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: green, blue-green, white.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: botryoidal.
Often confused with
Isle Royale Greenstone vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside isle royale greenstone
Minerals reported to co-occur with isle royale greenstone. 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
- Cleavage
- Distinct
- Rarity
- Rare
- Uses
- Gemstone, Lapidary, Collector
- Host rock
- Basaltic Lava Flows
- Typical price
- $20-100 per gram for high-quality polished stones
Where rockhounds find isle royale greenstone
Classic 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 isle royale greenstone typically forms. If you start seeing copper, quartz, calcite in float, you are in the right ground. Field specimens usually show a botryoidal habit, so train your eye for that shape before scanning the outcrop.
Common questions
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Find isle royale greenstone on the map
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