Thomsonite is a member of the zeolite group most prized by collectors for its beautiful, radiating concentric patterns often found in amygdules of volcanic rock. It is typically found in dense, botryoidal masses that polish well for lapidary work, showing striking eye-like motifs.
Is this thomsonite?
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 thomsonite with a known reference. Thomsonite sits at Mohs 5-5.5 — softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Thomsonite leaves a white streak.
- 3Read the lusterHold the specimen under a strong light. Thomsonite typically shows a vitreous luster.
- 4Match the color rangeCompare against the expected color range: white, pink, green, yellow, brown, colorless.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: botryoidal, radiating fibrous, bladed crystals.
Often confused with
Thomsonite vs. its common look-alikes — and how to tell them apart in the field.
Often found alongside thomsonite
Minerals reported to co-occur with thomsonite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- NaCa₂Al₅Si₅O₂₀·6H₂O
- Mohs hardness
- 5-5.5
- Density
- 2.3-2.4 g/cm³
- Streak
- White
- Luster
- Vitreous
- Transparency
- Translucent
- Crystal system
- Orthorhombic
- Crystal habit
- Botryoidal, Radiating Fibrous, Bladed Crystals
- Cleavage
- Perfect in One Direction
- Rarity
- Uncommon
- Uses
- Collector, Lapidary
- Host rock
- Basaltic Lava Amygdales
- Typical price
- $10-100 for cabochons and cabinet specimens
Where rockhounds find thomsonite
8 mapped spotsClassic worldwide localities
- Grand Marais, Minnesota
- Keweenaw Peninsula, Michigan
- Fayoum, Egypt
- Dunbar, Scotland
- Nova Scotia, Canada
Field-hunting tip
Look in basaltic lava amygdales country — that is the host setting where thomsonite typically forms. If you start seeing prehnite, datolite, calcite in float, you are in the right ground. Field specimens usually show a botryoidal, radiating fibrous, bladed crystals habit, so train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Michigan, Minnesota, Oregon — start trip planning there.
Common questions
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Find thomsonite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
