Bornhardtite is a rare cobalt selenide mineral typically occurring as microscopic inclusions in other selenide minerals. It is best identified in laboratory settings via X-ray diffraction or electron microprobe analysis due to its visual similarity to other metallic sulfides and selenides.

Hardness

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrareLinnaeite Group family

Bornhardtite

Hardness: ~4
Luster: Metallic
Streak: Black
Transparency: Opaque

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Is this bornhardtite?

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 bornhardtite with a known reference. Bornhardtite sits at Mohs ~4, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: black.
3
Read the luster
Hold the specimen under a strong light. Luster: metallic.
4
Match the color range
Compare against the expected color range: copper, pink.
5
Look at form & habit
Crystal system: cubic. Typical habit: massive, granular, rarely octahedral.

Often confused with

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

mineral

Linnaeite

Mohs 4.5-5.5

How to tell apart: Streak differs, so Bornhardtite leaves black, Linnaeite leaves grayish black.

Often found alongside bornhardtite

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

All properties

Chemical formula: Co₃Se₄
Mohs hardness: ~4
Density: 6.166 g/cm³
Colors: CopperPink
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Cubic
Crystal habit: Massive, granular, rarely octahedral
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Of hydrothermal origin

Where rockhounds find bornhardtite

Classic worldwide localities

Harz Mountains, from the Trogtal quarry, near Lautenthal (type locality), and at Tilkerode, Germany
Pinky Fault uranium deposit, Saskatchewan, Canada
Cerro de Cacheuta, Mendoza Province, Argentina

Field-hunting tip

Where it forms: Of hydrothermal origin. If you start seeing trogtalite, hastite, clausthalite in float, you are in the right ground. Typical habit: massive, granular, rarely octahedral. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify bornhardtite?+

Mohs hardness is ~4. Luster: metallic. The streak is black. Common colors include copper, pink.

Where is bornhardtite found?+

Notable localities include Harz Mountains, from the Trogtal quarry, near Lautenthal (type locality), and at Tilkerode, Germany; Pinky Fault uranium deposit, Saskatchewan, Canada; Cerro de Cacheuta, Mendoza Province, Argentina.

Is bornhardtite safe to handle?+

Bornhardtite contains cobalt and selenium. Contains selenium, which is toxic if ingested or inhaled as dust; handle with care and wash hands after handling.

What rocks look like bornhardtite?+

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

What minerals are found with bornhardtite?+

Bornhardtite commonly co-occurs with Trogtalite, Hastite, Clausthalite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does bornhardtite form in?+

Where it forms: Of hydrothermal origin. Working float back to the host body is the standard way to chase a fresh occurrence.

What is bornhardtite used for?+

Bornhardtite is used in collector.

Find bornhardtite on the map

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