Sederholmite is a rare nickel selenide that typically occurs as tiny, metallic, hexagonal plates or grains within hydrothermal veins. It is most often identified by its association with other rare selenium-bearing minerals in complex ore deposits. Collectors primarily seek it as an obscure end-member in mineralogical suites.

Hardness

2.5-3

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrareMillerite Group family

Sederholmite

Hardness: 2.5-3
Luster: Metallic
Streak: Black
Transparency: Opaque

Is this sederholmite?

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 sederholmite with a known reference. Sederholmite sits at Mohs 2.5-3, 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: yellow, orange yellow.
5
Look at form & habit
Crystal system: hexagonal. Typical habit: inclusions.

Often confused with

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

mineral

Millerite

Mohs 3-3.5

How to tell apart: Streak differs, so Sederholmite leaves black, Millerite leaves greenish black.

How to tell apart: Sederholmite is noticeably harder (Mohs 2.5-3 vs. 1).

Compare →

Often found alongside sederholmite

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

All properties

Chemical formula: β-NiSe
Mohs hardness: 2.5-3
Density: 7.06 g/cm³
Colors: Yellow Orange yellow
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Hexagonal
Crystal habit: Inclusions
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: In calcite veins, in sills of albite diabase in schist, associated with low-grade uranium mineralization

Where rockhounds find sederholmite

Classic worldwide localities

Kuusamo, northeastern Finland (type locality)

Field-hunting tip

Where it forms: In calcite veins, in sills of albite diabase in schist, associated with low-grade uranium mineralization. If you start seeing wilkmanite, penroseite, clausthalite in float, you are in the right ground. Typical habit: inclusions. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify sederholmite?+

Mohs hardness is 2.5-3. Luster: metallic. The streak is black. Common colors include yellow, orange yellow.

Where is sederholmite found?+

Notable localities include Kuusamo, northeastern Finland (type locality).

Is sederholmite safe to handle?+

Sederholmite contains nickel and selenium. Contains nickel and selenium, both of which are toxic if ingested or inhaled as dust; wash hands thoroughly after handling and avoid creating dust.

What rocks look like sederholmite?+

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

What minerals are found with sederholmite?+

Sederholmite commonly co-occurs with Wilkmanite, Penroseite, Clausthalite, Calcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does sederholmite form in?+

Where it forms: In calcite veins, in sills of albite diabase in schist, associated with low-grade uranium mineralization. Working float back to the host body is the standard way to chase a fresh occurrence.

What is sederholmite used for?+

Sederholmite is used in collector.

Find sederholmite on the map

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