Thomsonite-Ca is best known for its attractive, radial, multicolored fan or pea-like patterns found within basalt amygdules. Collectors prize it for its unique fibrous structure that displays beautiful chatoyancy or banding when polished by lapidaries. It is most frequently found in volcanic rocks, particularly within Lake Superior amygdaloids.

Hardness

5-5.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineraluncommonZeolite Group family

Thomsonite-Ca

Hardness: 5-5.5
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this thomsonite-ca?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch thomsonite-ca with a known reference. Thomsonite-Ca sits at Mohs 5-5.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Thomsonite-Ca leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Thomsonite-Ca typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, pink, green, yellow, brown, colorless.
5
Look at form & habit
Crystal system: orthorhombic. Typical habit: radial fibrous, spherical, or botryoidal clusters.

Often confused with

Thomsonite-Ca vs. its common look-alikes — and how to tell them apart in the field.

Often found alongside thomsonite-ca

Minerals reported to co-occur with thomsonite-ca. 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
Colors: White Pink Green Yellow Brown Colorless
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Orthorhombic
Crystal habit: Radial Fibrous, Spherical, Or Botryoidal Clusters
Cleavage: Perfect
Rarity: Uncommon
Uses: Collector, Lapidary, Decorative
Host rock: Basaltic Amygdaloidal Cavities and Pegmatites
Typical price: $10-50 for small nodules, $100+ for high-quality polished cabochons

Where rockhounds find thomsonite-ca

Classic worldwide localities

Grand Marais, Minnesota, USA
Keweenaw Peninsula, Michigan, USA
Ice River, British Columbia, Canada
Seydisfjordur, Iceland
Scotland

Field-hunting tip

Look in basaltic amygdaloidal cavities and pegmatites country — that is the host setting where thomsonite-ca typically forms. If you start seeing prehnite, calcite, datolite in float, you are in the right ground. Field specimens usually show a radial fibrous, spherical, or botryoidal clusters habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify thomsonite-ca?+

Mohs hardness is 5-5.5. It typically shows a vitreous luster. The streak is white. Common colors include white, pink, green, yellow.

Where is thomsonite-ca found?+

Notable localities include Grand Marais, Minnesota, USA; Keweenaw Peninsula, Michigan, USA; Ice River, British Columbia, Canada; Seydisfjordur, Iceland; Scotland.

How much is thomsonite-ca worth?+

Typical asking prices fall in the range of $10-50 for small nodules, $100+ for high-quality polished cabochons. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like thomsonite-ca?+

Thomsonite-Ca is most often confused with Mesolite, Natrolite, Scolecite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with thomsonite-ca?+

Thomsonite-Ca commonly co-occurs with Prehnite, Calcite, Datolite, Analcime. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does thomsonite-ca form in?+

Thomsonite-Ca typically forms in basaltic amygdaloidal cavities and pegmatites. Working float back to the host body is the standard way to chase a fresh occurrence.

What is thomsonite-ca used for?+

Thomsonite-Ca is used in collector, lapidary, decorative.

Find thomsonite-ca on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.