Kusachiite is a very rare copper bismuth oxide mineral first identified in the Kusachi mine in Japan. It typically occurs as tiny, dark-colored, tabular crystals within metasomatized limestone skarn deposits.

Hardness

3-4

Mohs

Luster

Subadamantine

Streak

Brown

Transparency

Opaque

mineralrare

Kusachiite

Hardness: 3-4
Luster: Subadamantine
Streak: Brown
Transparency: Opaque

Is this kusachiite?

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 kusachiite with a known reference. Kusachiite sits at Mohs 3-4, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Kusachiite leaves a brown streak.
3
Read the luster
Hold the specimen under a strong light. Kusachiite typically shows a subadamantine luster.
4
Match the color range
Compare against the expected color range: black, dark brown.
5
Look at form & habit
Crystal system: tetragonal. Typical habit: tabular crystals, aggregates.

Often confused with

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

mineral

Tenorite

Mohs 3-4

How to tell apart: Streak differs, so Kusachiite leaves brown, Tenorite leaves black; luster reads subadamantine on Kusachiite and metallic on Tenorite.

How to tell apart: Streak differs, so Kusachiite leaves brown, Bismite leaves white; luster reads subadamantine on Kusachiite and adamantine on Bismite.

Compare →

Often found alongside kusachiite

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

All properties

Chemical formula: CuBi₂O₄
Mohs hardness: 3-4
Density: 6.2 g/cm³
Colors: Black Dark Brown
Streak: Brown
Luster: Subadamantine
Transparency: Opaque
Crystal system: Tetragonal
Crystal habit: Tabular Crystals, Aggregates
Cleavage: None
Rarity: Rare
Uses: Collector
Host rock: Metasomatized Limestone
Typical price: $50-300 per specimen depending on size

Where rockhounds find kusachiite

Classic worldwide localities

Kusachi mine, Gunma Prefecture, Japan

Field-hunting tip

Look in metasomatized limestone country, the host setting where kusachiite typically forms. If you start seeing kuselite, bismite, copper in float, you are in the right ground. Field specimens usually show a tabular crystals, aggregates habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify kusachiite?+

Mohs hardness is 3-4. It typically shows a subadamantine luster. The streak is brown. Common colors include black, dark brown.

Where is kusachiite found?+

Notable localities include Kusachi mine, Gunma Prefecture, Japan.

How much is kusachiite worth?+

Typical asking prices fall in the range of $50-300 per specimen depending on size. Quality, size, and provenance can move individual specimens well outside that range.

Is kusachiite safe to handle?+

It contains toxic constituents. Contains Bismuth and Copper; avoid inhalation of dust and wash hands thoroughly after handling specimens. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like kusachiite?+

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

What minerals are found with kusachiite?+

Kusachiite commonly co-occurs with Kuselite, Bismite, Copper, Calcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does kusachiite form in?+

Kusachiite typically forms in metasomatized limestone. Working float back to the host body is the standard way to chase a fresh occurrence.

What is kusachiite used for?+

Kusachiite is used in collector.

Find kusachiite on the map

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