Thaumasite is a rare hydrated calcium silicate sulfate carbonate known for its unusual chemical composition. It typically forms delicate needle-like or prismatic crystals in vugs and cavities, often associated with other calcium-rich minerals. Collectors value it for its complex crystal habits and its tendency to be found in altered basalt or metamorphic environments.

Hardness

3.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineraluncommon

Thaumasite

Hardness: 3.5
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this thaumasite?

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 thaumasite with a known reference. Thaumasite sits at Mohs 3.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Thaumasite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Thaumasite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: white, colorless.
5
Look at form & habit
Crystal system: hexagonal. Typical habit: prismatic crystals, acicular, fibrous, massive.

Often confused with

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

mineral

Gypsum

Mohs 2

How to tell apart: Thaumasite is noticeably harder (Mohs 3.5 vs. 2).

How to tell apart: Thaumasite is noticeably harder (Mohs 3.5 vs. 2).

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Often found alongside thaumasite

Minerals reported to co-occur with thaumasite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: Ca₃Si(OH)₆(CO₃)(SO₄)·12H₂O
Mohs hardness: 3.5
Density: 1.49-1.51 g/cm³
Colors: White Colorless
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Hexagonal
Crystal habit: Prismatic Crystals, Acicular, Fibrous, Massive
Cleavage: Perfect
Rarity: Uncommon
Uses: Collector, Scientific Research
Host rock: Metamorphic Skarns, Hydrothermal Veins in Basalt, Sulfate-rich Environments
Typical price: $10-100 per specimen depending on size and quality

Where rockhounds find thaumasite

Classic worldwide localities

Sweden
South Africa
Canada
USA
Russia

Field-hunting tip

Look in metamorphic skarns, hydrothermal veins in basalt, sulfate-rich environments country — that is the host setting where thaumasite typically forms. If you start seeing calcite, ettringite, apophyllite in float, you are in the right ground. Field specimens usually show a prismatic crystals, acicular, fibrous, massive habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify thaumasite?+

Mohs hardness is 3.5. It typically shows a vitreous luster. The streak is white. Common colors include white, colorless.

Where is thaumasite found?+

Notable localities include Sweden; South Africa; Canada; USA; Russia.

How much is thaumasite worth?+

Typical asking prices fall in the range of $10-100 per specimen depending on size and quality. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like thaumasite?+

Thaumasite is most often confused with Gypsum, Ettringite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with thaumasite?+

Thaumasite commonly co-occurs with Calcite, Ettringite, Apophyllite, Prehnite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does thaumasite form in?+

Thaumasite typically forms in metamorphic skarns, hydrothermal veins in basalt, sulfate-rich environments. Working float back to the host body is the standard way to chase a fresh occurrence.

What is thaumasite used for?+

Thaumasite is used in collector, scientific research.

Find thaumasite on the map

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