Riebeckite is an amphibole group mineral typically found in silica-rich alkaline igneous rocks like granite. It often occurs as prismatic crystals or as a fibrous, asbestiform variety known as crocidolite, which can replace other minerals during pseudomorphosis.

Hardness

Mohs

Luster

Vitreous to silky

Streak

Greenish brown

Transparency

Semitransparent

mineralcommonAmphibole Group family

Riebeckite

Hardness: 6
Luster: Vitreous to silky
Streak: Greenish brown
Transparency: Semitransparent

Is this riebeckite?

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 riebeckite with a known reference. Riebeckite sits at Mohs 6, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: greenish brown.
3
Read the luster
Hold the specimen under a strong light. Luster: vitreous to silky.
4
Match the color range
Compare against the expected color range: dark blue, black, blue-green, blue, dark green.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: fibrous, massive, striated.

Often confused with

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

mineral

Arfvedsonite

Mohs 5-6

How to tell apart: Streak differs, so Riebeckite leaves greenish brown, Arfvedsonite leaves deep bluish gray, gray-green.

How to tell apart: Streak differs, so Riebeckite leaves greenish brown, Glaucophane leaves blue-gray.

Compare →

Often found alongside riebeckite

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

All properties

Chemical formula: Na₂Fe²⁺₃Fe³⁺₂Si₈O₂₂(OH)₂
Mohs hardness: 6
Density: 3.28-3.44 g/cm³
Colors: Dark blue Black Blue-green Blue Dark green
Streak: Greenish brown
Luster: Vitreous to silky
Transparency: Semitransparent
Crystal system: Monoclinic
Crystal habit: Fibrous, massive, striated
Cleavage: Perfect on {110}, intersecting at 56° and 124°; partings on {100}, {010}
Rarity: Common
Uses: Collector, Lapidary
Host rock: In alkalic granites and syenites; rarer in felsic volcanics and granite pegmatites; in some schists

Where rockhounds find riebeckite

1 mapped spots

Classic worldwide localities

Quincy, Norfolk Co., Massachusetts, USA
St. Peters Dome, near Pikes Peak, El Paso Co., Colorado, USA
Pass, Okanogan Co., Washington, USA
Socotra Island, Indian Ocean
North from Koegas, Cape Province, and east of Pietersburg, Transvaal, South Africa
Sultan Hamud, Kenya

Field-hunting tip

Where it forms: In alkalic granites and syenites; rarer in felsic volcanics and granite pegmatites; in some schists. If you start seeing aegirine, nepheline, albite in float, you are in the right ground. Typical habit: fibrous, massive, striated. Train your eye for that shape before scanning the outcrop. In the U.S., the densest reported localities are in Utah. Start trip planning there.

Browse 1 U.S. spots for riebeckite →

Common questions

How do you identify riebeckite?+

Mohs hardness is 6. Luster: vitreous to silky. The streak is greenish brown. Common colors include dark blue, black, blue-green, blue.

Where is riebeckite found?+

Notable localities include Quincy, Norfolk Co., Massachusetts, USA; St. Peters Dome, near Pikes Peak, El Paso Co., Colorado, USA; Pass, Okanogan Co., Washington, USA; Socotra Island, Indian Ocean; North from Koegas, Cape Province, and east of Pietersburg, Transvaal, South Africa.

Can I find riebeckite in the United States?+

RockHoundR maps 1 riebeckite rockhounding spots across 1 U.S. states. The top states are Utah.

Is riebeckite safe to handle?+

Riebeckite contains asbestiform fibers. The asbestiform variety, crocidolite, is a known carcinogen and respiratory hazard; avoid inhaling fibers during handling or breaking of specimens.

What rocks look like riebeckite?+

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

What minerals are found with riebeckite?+

Riebeckite commonly co-occurs with Aegirine, Nepheline, Albite, Arfvedsonite, Tremolite, Ferro-actinolite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does riebeckite form in?+

Where it forms: In alkalic granites and syenites; rarer in felsic volcanics and granite pegmatites; in some schists. Working float back to the host body is the standard way to chase a fresh occurrence.

What is riebeckite used for?+

Riebeckite is used in collector, lapidary.

Find riebeckite on the map

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