Rumoiite is a rare gold tin mineral that forms silver white irregular grains. It occurs in fluvial placer deposits. Associated minerals include gold, shosanbetsuite and yuanjiangite. It was first described from the Shosanbetsu river, Rumoi province, Hokkaido, Japan.
Is this rumoiite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch rumoiite with a known reference. Rumoiite sits at Mohs 2.5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Rumoiite leaves a gray streak.
- 3Read the lusterHold the specimen under a strong light. Rumoiite typically shows a metallic luster.
- 4Match the color rangeCompare against the expected color range: silver white, white.
- 5Look at form & habitCrystal system: orthorhombic. Typical habit: irregular grains.
Often confused with
Rumoiite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Sperrylite is the harder of the two (Mohs 6-7 vs. 2.5); streak differs, so Rumoiite leaves gray, Sperrylite leaves black.
How to tell apart: Cooperite is the harder of the two (Mohs 4-5 vs. 2.5); streak differs, so Rumoiite leaves gray, Cooperite leaves black.
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
- AuSn₂
- Mohs hardness
- 2.5
- Density
- 10.1 g/cm³
- Colors
- Streak
- Gray
- Luster
- Metallic
- Transparency
- Opaque
- Crystal system
- Orthorhombic
- Crystal habit
- Irregular Grains
- Cleavage
- None
- Rarity
- Rare
- Uses
- Collector, Scientific Research
- Host rock
- Hydrothermal Veins in Ultrabasic Rocks
Where rockhounds find rumoiite
Classic worldwide localities
- Rumoi, Hokkaido, Japan
Field-hunting tip
Look in hydrothermal veins in ultrabasic rocks country, 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 irregular grains habit, so train your eye for that shape before scanning the outcrop.
Common questions
How do you identify rumoiite?+
Where is rumoiite found?+
What rocks look like rumoiite?+
What minerals are found with rumoiite?+
What kind of rock does rumoiite form in?+
What is rumoiite used for?+
Find rumoiite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
