Geffroyite is a rare copper-iron-silver selenide mineral typically found as microscopic grains in hydrothermal vein systems. It is most commonly identified through polished section analysis in laboratory settings alongside other selenide minerals.

Hardness

2.5

Mohs

Luster

Metallic

Streak

Black

Transparency

Opaque

mineralrarePentlandite Group family

Geffroyite

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

Is this geffroyite?

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 geffroyite with a known reference. Geffroyite sits at Mohs 2.5, 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: bronze brown.
5
Look at form & habit
Crystal system: cubic. Typical habit: anhedral grains, massive.

Often confused with

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

mineral

Clausthalite

Mohs 2.5-3

How to tell apart: Streak differs, so Geffroyite leaves black, Clausthalite leaves grayish black.

Often found alongside geffroyite

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

All properties

Chemical formula: (Cu,Fe,Ag)₉Se₈
Mohs hardness: 2.5
Density: 5.39 g/cm³
Colors: Bronze brown
Streak: Black
Luster: Metallic
Transparency: Opaque
Crystal system: Cubic
Crystal habit: Anhedral grains, massive
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: As late-stage deposits in veins cutting granite (Chaméane mine, France)

Where rockhounds find geffroyite

Classic worldwide localities

Chaméane uranium mine, near Vernet-la-Varenne, Puy-de-Dôme, France (type locality)
San Miguel prospect, 10 km northwest of the Moctezuma (Bambolla) mine, 12 km south of Moctezuma, Sonora, Mexico

Field-hunting tip

Where it forms: As late-stage deposits in veins cutting granite (Chaméane mine, France). If you start seeing eskebornite, clausthalite, chaméanite in float, you are in the right ground. Typical habit: anhedral grains, massive. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify geffroyite?+

Mohs hardness is 2.5. Luster: metallic. The streak is black. Common colors include bronze brown.

Where is geffroyite found?+

Notable localities include Chaméane uranium mine, near Vernet-la-Varenne, Puy-de-Dôme, France (type locality); San Miguel prospect, 10 km northwest of the Moctezuma (Bambolla) mine, 12 km south of Moctezuma, Sonora, Mexico.

Is geffroyite safe to handle?+

Geffroyite contains copper and selenium. Contains selenium, which can be toxic if inhaled as dust or ingested. Handle with care and wash hands thoroughly after handling.

What rocks look like geffroyite?+

Geffroyite is most often confused with Clausthalite, Penroseite, Athabascaite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with geffroyite?+

Geffroyite commonly co-occurs with Eskebornite, Clausthalite, Chaméanite, Ankerite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does geffroyite form in?+

Where it forms: As late-stage deposits in veins cutting granite (Chaméane mine, France). Working float back to the host body is the standard way to chase a fresh occurrence.

What is geffroyite used for?+

Geffroyite is used in collector.

Find geffroyite on the map

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