Cuprostibite is an extremely rare copper antimony mineral typically found as small, non-descript grains within alkaline igneous rocks. It is primarily identified in lab settings due to its subtle metallic appearance and association with exotic minerals in the Ilimaussaq complex.

Hardness

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrare

Cuprostibite

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

Is this cuprostibite?

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 cuprostibite with a known reference. Cuprostibite 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: steel gray.
5
Look at form & habit
Crystal system: tetragonal. Typical habit: aggregates.

Often confused with

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

Often found alongside cuprostibite

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

All properties

Chemical formula: Cu₂(Sb,Tl)
Mohs hardness: 4
Density: 8.42 g/cm³
Colors: Steel gray
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Tetragonal
Crystal habit: Aggregates
Cleavage: One direction
Rarity: Rare
Uses: Collector
Host rock: As fine-grained aggregates in a vein of ussingite cutting sodalite syenite (Ilímaussaq intrusion, Greenland); in sulfide veins in metamorphic siliceous dolostone (Långsjön, Sweden)

Where rockhounds find cuprostibite

Classic worldwide localities

Franklin, Sussex Co., New Jersey, USA
Southern Greenland, from Mt. Nákâlâq (type locality), northwest of Taseq Lake
Långsjön, at Långban, and in the Getberg mine, near Långban, Värmland, Sweden

Field-hunting tip

Where it forms: As fine-grained aggregates in a vein of ussingite cutting sodalite syenite (Ilímaussaq intrusion, Greenland); in sulfide veins in metamorphic siliceous dolostone (Långsjön, Sweden). If you start seeing löllingite, antimonian silver, chalcopyrite in float, you are in the right ground. Typical habit: aggregates. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify cuprostibite?+

Mohs hardness is 4. Luster: metallic. The streak is black. Common colors include steel gray.

Where is cuprostibite found?+

Notable localities include Franklin, Sussex Co., New Jersey, USA; Southern Greenland, from Mt. Nákâlâq (type locality), northwest of Taseq Lake; Långsjön, at Långban, and in the Getberg mine, near Långban, Värmland, Sweden.

Is cuprostibite safe to handle?+

Cuprostibite contains antimony, copper and thallium and calls for real care. Contains antimony and copper; handle with care and wash hands after handling to avoid ingestion or skin irritation.

What rocks look like cuprostibite?+

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

What minerals are found with cuprostibite?+

Cuprostibite commonly co-occurs with Löllingite, Antimonian silver, Chalcopyrite, Chalcothallite, Chalcocite, Bismuth. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does cuprostibite form in?+

Where it forms: As fine-grained aggregates in a vein of ussingite cutting sodalite syenite (Ilímaussaq intrusion, Greenland); in sulfide veins in metamorphic siliceous dolostone (Långsjön, Sweden). Working float back to the host body is the standard way to chase a fresh occurrence.

What is cuprostibite used for?+

Cuprostibite is used in collector.

Find cuprostibite on the map

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