Rumoiite is a rare platinum tin intermetallic mineral first described from the Rumoi district in Japan. It typically occurs as minute metallic grains within complex hydrothermal sulfide deposits associated with mafic to ultramafic complexes. Due to its extreme rarity, it is almost exclusively a specimen for specialized systematic mineral collections.

Hardness

3.5

Mohs

Luster

Metallic

Streak

Grey

Transparency

Opaque

mineralrare

Rumoiite

Hardness: 3.5
Luster: Metallic
Streak: Grey
Transparency: Opaque

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Is this rumoiite?

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 rumoiite with a known reference. Rumoiite sits at Mohs 3.5 — softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Rumoiite leaves a grey streak.
3
Read the luster
Hold the specimen under a strong light. Rumoiite typically shows a metallic luster.
4
Match the color range
Compare against the expected color range: white, gray.
5
Look at form & habit
Crystal system: hexagonal. Typical habit: anhedral grains.

Often confused with

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

mineral

Sperrylite

Mohs 6-7

How to tell apart: Sperrylite is the harder of the two (Mohs 6-7 vs. 3.5); streak differs — Rumoiite leaves grey, Sperrylite leaves black.

How to tell apart: Streak differs — Rumoiite leaves grey, Cooperite leaves black.

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Often found alongside rumoiite

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

All properties

Chemical formula: PtSn
Mohs hardness: 3.5
Density: 9.43 g/cm³
Colors: White Gray
Streak: Grey
Luster: Metallic
Transparency: Opaque
Crystal system: Hexagonal
Crystal habit: Anhedral Grains
Cleavage: None
Rarity: Rare
Uses: Collector, Scientific Research
Host rock: Hydrothermal Veins in Ultrabasic Rocks
Typical price: $100-500 per micro-specimen

Where rockhounds find rumoiite

Classic worldwide localities

Rumoi, Hokkaido, Japan

Field-hunting tip

Look in hydrothermal veins in ultrabasic rocks country — that is the host setting where rumoiite typically forms. If you start seeing platinum, bornite, chalcopyrite in float, you are in the right ground. Field specimens usually show a anhedral grains habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify rumoiite?+

Mohs hardness is 3.5. It typically shows a metallic luster. The streak is grey. Common colors include white, gray.

Where is rumoiite found?+

Notable localities include Rumoi, Hokkaido, Japan.

How much is rumoiite worth?+

Typical asking prices fall in the range of $100-500 per micro-specimen. Quality, size, and provenance can move individual specimens well outside that range.

Is rumoiite safe to handle?+

It contains toxic constituents. Contains tin and platinum; handle with care to avoid ingestion or inhalation of dust. Wash hands thoroughly after handling. Handle with care, avoid grinding or breathing dust, and store separately.

What rocks look like rumoiite?+

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

What minerals are found with rumoiite?+

Rumoiite commonly co-occurs with Platinum, Bornite, Chalcopyrite, Galena, Sphalerite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does rumoiite form in?+

Rumoiite typically forms in hydrothermal veins in ultrabasic rocks. Working float back to the host body is the standard way to chase a fresh occurrence.

What is rumoiite used for?+

Rumoiite is used in collector, scientific research.

Find rumoiite on the map

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