Grunerite is a cummingtonite-subgroup amphibole known for its association with iron-rich metamorphic rocks. It often presents in fibrous or radiating columnar masses and is most famous for its fibrous variety, amosite, historically used as a commercial form of asbestos.

Hardness

5-6

Mohs

Luster

Vitreous

Streak

White

Transparency

Translucent

mineralcommonAmphibole Group family

Grunerite

Hardness: 5-6
Luster: Vitreous
Streak: White
Transparency: Translucent

Is this grunerite?

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 grunerite with a known reference. Grunerite sits at Mohs 5-6, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Grunerite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Grunerite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: brown, greenish-brown, gray, dark green.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: fibrous, columnar, or massive aggregates.

Often confused with

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

How to tell apart: Luster reads vitreous on Grunerite and vitreous to silky on Actinolite.

Often found alongside grunerite

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

All properties

Chemical formula: Fe₇Si₈O₂₂(OH)₂
Mohs hardness: 5-6
Density: 3.4-3.6 g/cm³
Colors: Brown Greenish-brown Gray Dark Green
Streak: White
Luster: Vitreous
Transparency: Translucent
Crystal system: Monoclinic
Crystal habit: Fibrous, Columnar, Or Massive Aggregates
Cleavage: Perfect in Two Directions
Rarity: Common
Uses: Collector, Scientific Study
Host rock: Metamorphosed Iron Formations
Typical price: $10-100 depending on specimen size and quality

Where rockhounds find grunerite

Classic worldwide localities

Ontario, Canada
Minnesota, USA
France
Japan
Switzerland

Field-hunting tip

Look in metamorphosed iron formations country, the host setting where grunerite typically forms. If you start seeing magnetite, quartz, hematite in float, you are in the right ground. Field specimens usually show a fibrous, columnar, or massive aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify grunerite?+

Mohs hardness is 5-6. It typically shows a vitreous luster. The streak is white. Common colors include brown, greenish-brown, gray, dark green.

Where is grunerite found?+

Notable localities include Ontario, Canada; Minnesota, USA; France; Japan; Switzerland.

How much is grunerite worth?+

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

Is grunerite safe to handle?+

It contains toxic constituents. Grunerite can occur in asbestiform habits; avoid inhalation of fibers as they are hazardous and can cause lung damage or disease. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like grunerite?+

Grunerite is most often confused with Cummingtonite, Actinolite, Anthophyllite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with grunerite?+

Grunerite commonly co-occurs with Magnetite, Quartz, Hematite, Siderite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does grunerite form in?+

Grunerite typically forms in metamorphosed iron formations. Working float back to the host body is the standard way to chase a fresh occurrence.

What is grunerite used for?+

Grunerite is used in collector, scientific study.

Find grunerite on the map

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